PURPOSE: Men on active surveillance (AS) for prostate cancer are extremely interested in dietary changes or supplements to prevent progression of their disease. We sought to determine whether a high omega-3, low omega-6 fatty acid diet with fish oil capsules (D + FO) decreases proliferation (Ki-67) in prostate biopsies in men with prostate cancer on AS over a 1-year time period. METHODS: In this phase II, prospective randomized trial, men (N = 100) with grade group 1 or 2 prostate cancer who elected AS were randomly assigned to the D + FO or a control group. Same-site prostate biopsies were obtained at baseline and 1 year. The primary end point was the change in Ki-67 index from baseline to 1 year from same-site biopsies compared between the groups. RESULTS: The Ki-67 index decreased in the D + FO group by approximately 15% from baseline to 1 year (1.34% at baseline, 1.14% at 1 year) and increased in the control group by approximately 24% from baseline to 1 year (1.23% at baseline, 1.52% at 1 year), resulting in a statistically significant difference in the change of Ki-67 index between the groups (95% CI, 2% to 52%, CONCLUSION: A high omega-3, low omega-6 diet with FO for 1 year resulted in a significant reduction in Ki-67 index, a biomarker for prostate cancer progression, metastasis, and death. These findings support future phase III trials incorporating this intervention in men on AS.