Prostate specific antigen (PSA) and PSA-density (PSAd) were studied in 120 symptomatic patients with benign prostatic hyperplasia at digital rectal examination. PSA and PSAd were measured before and 3 months after transurethral resection. Comparisons were made between patients in whom histologic examination showed benign hyperplasia and those with histologically demonstrated prostatic cancer. The influence of age and of serum testosterone on PSA and PSAd was determined. PSAd in benign prostatic hyperplasia was the same in the transition zone (the adenomas) and the peripheral zone (mean 0.065), indicating no increased synthesis or leakage of PSA. PSAd with cut-off value 0.10 ng x ml-1 x cc-1 showed higher sensitivity and positive predictive value (75% vs 50% and 0.33 vs 0.15, respectively) in nonpalpable prostatic cancer than did PSA with cut-off at 4 ng x ml-1. No statistically significant influence of testosterone and/or age on serum-PSA or PSAd was found.