The short answer
Ki-67 is a marker of how quickly cancer cells are dividing. A higher percentage means more cells are actively growing, which can suggest a faster-growing tumor.
Ki-67 measures the share of cancer cells that are actively dividing.
It is reported as a percentage — higher means more cells are growing.
A higher score can suggest a faster-growing tumor.
It is used alongside other findings, not on its own.
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The full explanation.
What Ki-67 measures
Ki-67 is a protein. It appears inside cells while they are actively dividing. Resting cells do not make it. A pathology test measures what percent of your cancer cells contain this protein. That percentage is your Ki-67 score, sometimes called the Ki-67 index.
Reading the number
The score is written as a percentage, from 0% to 100%. A low number means few cells were dividing at that moment. A high number means many were. A higher score often points to a faster-growing tumor. This is one clue doctors use when judging how a cancer may behave.
Why there is no single cutoff
What counts as "high" depends on the cancer type. In breast cancer, there is no single agreed number that separates low from high. Different labs use different thresholds, and studies show substantial disagreement between readers scoring the same sample in the 5% to 30% range. This is a known limitation of the test. Its usefulness for guiding treatment decisions has been debated for years because of these technical questions.
In some other cancers, Ki-67 is used with a clearer, tested cutoff. Neuroendocrine tumors are graded using a World Health Organization system built partly around Ki-67. A score under 3% points to Grade 1, the slowest-growing group. A score of 3% up to 20% points to Grade 2. A score over 20% points to Grade 3, the fastest-growing, most aggressive group. This system also uses a second measurement, the mitotic count, alongside Ki-67. When the two disagree, doctors use the higher grade.
What it changes
In breast cancer, Ki-67 is one of several inputs. It is not usually decisive on its own, since the lack of a standard cutoff makes it hard to build a firm treatment rule around. In neuroendocrine tumors, the Ki-67-based grade matters much more directly. Low-grade tumors are often watched, or treated with milder options like somatostatin analogs. High-grade tumors typically need more intensive treatment, such as targeted therapy or chemotherapy.
What it does not tell you
A single Ki-67 number does not, by itself, tell you the stage of your cancer. It does not tell you whether it has spread. It also should not be compared directly across different labs or cancer types, since methods and thresholds vary. Ki-67 works best when read alongside the tumor's grade, stage, and other biomarkers, not on its own.
Is this urgent?
Ki-67 results are part of routine pathology, not an emergency finding. What matters more is how your specific care team weighs it. If your report shows a Ki-67 score, ask your oncologist how it factors into your treatment plan, since the same number can mean different things in different cancers.
What to ask your team
- What was my Ki-67 score, and how is it used for my specific cancer type?
- Is there a standard cutoff used for my cancer, or is this read in context with other results?
- Does this score change how aggressively my cancer is being treated?
- Should this test be repeated if my treatment changes?
Where newer genomic tests fit in
Because Ki-67 alone is not tightly standardized in breast cancer, it is increasingly used alongside, rather than instead of, genomic tests such as Oncotype DX or MammaPrint. These tests look at a panel of genes rather than one protein, and they come with more rigorously validated cutoffs tied to actual treatment outcomes in large studies. If your Ki-67 result feels uncertain or borderline, ask whether a genomic test is appropriate for your specific cancer, since it may give a clearer answer than the Ki-67 percentage alone.
Watching how a tumor changes over time
In some situations, Ki-67 is measured more than once, for example before and after a short course of hormone therapy given ahead of surgery. A meaningful drop in Ki-67 after this kind of treatment can suggest the cancer is responding well to hormone-blocking drugs, which sometimes influences the plan for chemotherapy afterward. This kind of paired testing is more common in research settings and specialized centers than in routine care, so ask your oncologist directly if this approach is being used in your specific treatment plan.
Sources
- PubMed Central (Cureus) — Neuroendocrine Neoplasms of the Gastrointestinal Tract: Morphology, WHO 2022 Grading, and Prognostic Perspectives
- PubMed Central (Cancers) — Ki-67 as a Prognostic Biomarker in Invasive Breast Cancer
- Ki67 in Breast Cancer Assay — Canadian Association of Pathologists Task Force (PMC)
Words to know
Tap any term to see what it means.
Common questions
What is Ki-67?
Ki-67 is a protein found in cells that are actively dividing. A lab test measures the percentage of cancer cells that carry it, giving a sense of how fast the tumor is growing.
Is a high Ki-67 bad?
A higher percentage suggests more cells are dividing, which can mean a faster-growing tumor. But its meaning varies by cancer type and is weighed with other factors.
What is a normal Ki-67?
There is no single cutoff that applies to all cancers. The interpretation depends on the cancer type, so it is best understood with your doctor.
How is it used?
Ki-67 is one piece of information used with grade, stage, and other markers to understand a tumor's behavior and help plan treatment.
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Written by: Cancer ExplainedSources last checked: 2026-07-14 what this meansLast updated: 2026-08-11Next planned review: 2027-07-14
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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