Objectives. Dietary fat and fiber affect hormonal levels and may influence cancer progression. Flaxseed is a rich source of lignan and omega-3 fatty acids and may thwart prostate cancer. The potential effects of flaxseed may be enhanced with concomitant fat restriction. We undertook a pilot study to explore whether a flaxseed-supplemented, fat-restricted diet could affect the biomarkers of prostatic neoplasia. Methods. Twenty-five patients with prostate cancer who were awaiting prostatectomy were instructed on a low-fat (20% of kilocalories or less), flaxseed-supplemented (30 g/day) diet. The baseline and follow-up levels of prostate-specific antigen, testosterone, free androgen index, and total serum cholesterol were determined. The tumors of diet-treated patients were compared with those of historic cases (matched by age, race, prostate-specific antigen level at diagnosis, and biopsy Gleason sum) with respect to apoptosis (terminal deoxynucleotidyl transferase [TdT]-mediated dUTP-biotin nick end-labeling [TUNEL]) and proliferation (MIB-1). Results. The average duration on the diet was 34 days (range 21 to 77), during which time significant decreases were observed in total serum cholesterol (201 ± 39 mg/dL to 174 ± 42 mg/dL), total testosterone (422 ± 122 ng/dL to 360 ± 128 ng/dL), and free androgen index (36.3% ± 18.9% to 29.3% ± 16.8%) (all P <0.05). The baseline and follow-up levels of prostate-specific antigen were 8.1 ± 5.2 ng/mL and 8.5 ± 7.7 ng/mL, respectively, for the entire sample (P = 0.58); however, among men with Gleason sums of 6 or less (n = 19), the PSA values were 7.1 ± 3.9 ng/mL and 6.4 ± 4.1 ng/mL (P = 0.10). The mean proliferation index was 7.4 ± 7.8 for the historic controls versus 5.0 ± 4.9 for the diet-treated patients (P = 0.05). The distribution of the apoptotic indexes differed significantly (P = 0.01) between groups, with most historic controls exhibiting TUNEL categorical scores of 0; diet-treated patients largely exhibited scores of 1. Both the proliferation rate and apoptosis were significantly associated with the number of days on the diet (P = 0.049 and P = 0.017, respectively). Conclusions. These pilot data suggest that a flaxseed-supplemented, fat-restricted diet may affect prostate cancer biology and associated biomarkers. Further study is needed to determine the benefit of this dietary regimen as either a complementary or preventive therapy. Copyright © 2001 Elsevier Science Inc.