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Early diagnosis of prostate cancer is based on determination of serum prostate specific antigen (PSA) and digital rectal examination (DRE). In men with PSA values above 10 ng/ml and where there is a positive DRE, the indication for prostatic biopsy is given. However, there is controversy as to whether men with an intermediate PSA level of 4-10 ng/ml should undergo further evaluation. Since cancer secretes 12 times more serum PSA per volume than benign prostatic hyperplasia, PSA density (PSAD, serum PSA divided by the volume of the entire prostate) has been suggested as an additional diagnostic criterion. In 153 men with prostatism and PSA values between 4 and 10 ng/ml (Hybritech assay), the volume of the prostate was determined by transrectal ultrasonography and 6 systematic biopsies were performed. The groups with positive and negative biopsies respectively were compared according to age of the patient, results of DRE and PSAD. Histological grade of positive biopsies and pathological stage of patients who underwent radical prostatectomy were also compared with diagnostic criteria. Prostate cancer was found in 45 of 153 men (29.4%). Patient's age had no influence on detection rates. The positive predictive value of a PSAD of 0.2 or more was twice the value of a PSAD below 0.2, irrespective of DRE findings. An increasing number of positive biopsies was associated with increasing PSAD and histological grade. Only half of the tumors operated on were still confined to the prostate. Pathological stage could be predicted by the number of positive biopsies. We conclude that PSAD may be useful as a diagnostic parameter in men with prostatism and PSA values of 4-10 ng/ml. The prostate cancers that were detected in this setting are of clinical significance.