To enhance the ability of prostate-specific antigen (PSA) to distinguish benign prostatic hyperplasia (BPH) from cancer of the prostate (CaP) Benson et al. defined a new parameter, PSA density (PSAD), which is the ratio of the serum PSA concentration to the volume of the prostate. We have employed this parameter in a prospective study of 28 patients with clinically localised CaP and 57 patients with BPH. Mean PSAD was 0.116 for the BPH patients and 0.46 for the CaP patients (p < 0.005). The authors conclude that the information provided by PSAD is superior to absolute PSA values in the differentiation between BPH and CaP.