PURPOSE: We analyze patterns of prostate growth in men diagnosed with benign prostatic hyperplasia (BPH) and treated with placebo during 4 years, and determine which baseline parameters were the strongest predictors of growth. MATERIALS AND METHODS: A total of 3,040 men were enrolled in the 4-year randomized, placebo controlled Proscar Long-Term Efficacy and Safety study. Of these men a subgroup of 10% underwent pelvic magnetic resonance imaging prostate volume measurement at baseline and yearly thereafter. Absolute and percent volume changes during 4 years were calculated in the 164 placebo treated men in the subgroup. The ability of age, baseline prostate volume and prostate specific antigen (PSA) to predict prostate growth in placebo treated patients was assessed by multiple linear regression analyses, receiver operator characteristics curves, and evaluations of growth stratified by tertiles of baseline serum PSA and decades of life. RESULTS: In placebo treated patients a steady increase in mean plus or minus standard deviation prostate volume from year to year was noted (2.5+/-6.1, 4.9+/-6.8, 6.4+/-8.5 and 7.2+/-8.8 ml. at years 1, 2, 3 and 4, respectively). Mean volume changes at 4 years ranged from -9 to +30 ml. Mean percent change from baseline ranged from 12.5% to 16.6% for men 50 to 59 years old to those 70 to 79 years old. Baseline serum PSA was a strong predictor of growth with 7.4% to 22.0% change at 4 years from the lowest to highest PSA tertiles. Annualized growth rates from baseline were 0.7 ml. per year for PSA 0.2 to 1.3, 2.1 for PSA 1.4 to 3.2 and 3.3 for PSA 3.3 to 9.9 ng./ml. Multiple linear regression analysis showed that serum PSA was a stronger predictor of prostate growth than age or baseline prostate volume. All but 1 man with baseline serum PSA greater than 2.0 ng./ml. had prostate growth during 4 years, and 32.6% of men with serum PSA less than 2.0 exhibited a decrease in volume. CONCLUSIONS: Serum PSA is a stronger predictor of growth of the prostate in placebo treated patients than age or baseline prostate volume. Since prostate volume is a risk factor for acute urinary retention and the need for BPH related surgery, the ability of PSA to predict prostate growth may be an important factor when considering individual treatment options for BPH. Such use of PSA represents a shift in paradigm away from focusing solely on symptoms of BPH toward a more comprehensive approach with consideration of predicting and preventing risk factors of BPH related outcomes.