To evaluate the diagnostic usefulness of prostate-specific antigen density (PSAD) in prostate cancer (PC) prostate-specific antigen (PSA) concentrations were measured in 175 patients with benign prostatic hypertrophy (BPH) and 50 patients with PC. Patients with BPH were classified according to the presence of complications of the disease: urinary infection or the presence of a bladder catheter. PSAD levels were observed to be greater than 0.15 in 3% of the patients with uncomplicated BPH and in 40% of the patients with complicated BPH. PSA levels were higher than 10 micrograms/l in 3 and 27% of these patients, respectively. High levels of PSAD were observed in 80% of the patients with cancer. Sixty-four percent of the patients with cancer presented PSA levels greater than 10 micrograms/l. These results indicate that PSAD is a useful parameter in the differential diagnosis of PC and BPH with the diagnostic efficacy of PSAD being greater than that of the serum determination of PSA.