A total of 37 patients with well-documented benign prostatic hypertrophy (BPH) were referred to finasteride. In all subjects the prostate volume was > 60 cc. Serum total PSA (TPSA) and free/total PSA (%FPSA) values were recorded at 3-month intervals. After 6 months of treatment, the patients were divided into two groups in accordance with the numerical values of these two parameters. In the first group (25 patients), a drop in TPSA approached 50% reduction while the %FPSA level remained at the initial level. No malignancy was detected in these patients after 9 months of finasteride treatment and in 4-18 months additional follow-up. The second group (12 patients), consisted of subjects with a less pronounced decrease in TPSA concentration (ca. 28%) and a significant reduction in %FPSA mostly to values < 18% (cut-off point dividing BPH from cancer) during a 6-month monitoring period. During the extended part of the investigation, prostate cancer was diagnosed in 7 out of 11 of these latter patients (63.6%), or overall in 7 out of 30 (23.3%) patients who reached the end-point of the study. Accordingly, serial assessments of total and free PSA are necessary and sufficient clinical means to detect early prostate cancer in patients with a large benign prostate referred to finasteride.