We attempted to correlate prostate volume reduction in response to finasteride treatment with initial prostate-specific antigen (PSA) levels and PSA density in men with symptomatic benign prostatic hyperplasia (BPH). The average reductions in prostatic volume (transrectal ultrasonography) were 27% and 34% after 6 and 12 months of finasteride therapy, respectively. Serum PSA levels decreased by 45% (6 months) and 50% (12 months). There was a positive correlation between initial serum PSA values and initial prostate volumes (r = 0.57, P < 0.001). There was no correlation, however, between the initial serum PSA or PSA-density values and prostate volume reduction. These data indicate that initial serum PSA and PSA-density values are not predictive of the response to finasteride therapy in terms of prostate size reduction.