The usefulness of the fPSA fraction in the differential diagnosis of benign prostate hyperplasia (BPH) and prostate cancer was evaluated, with the aim of improving the diagnostic efficacy of PSA. Serum PSA and fPSA determinations were performed by an enzymoimmunoassay technique on an ES-300 system. fPSA constitutes a minor fraction both in normal subjects and in patients with prostate disease, being significantly lower in patients with untreated prostate cancer than in patients with BPH. Likewise, the authors have observed that the sensitivity of the fPSA/PSA ratio has an inversely proportional relationship with the stage of the disease. The results obtained in patients with PSA levels between 4 and 20 micrograms/l are also of note. In this series of patients, the efficacy of the fPSA/PSA ratio was higher than that of PSA, showing a sensitivity of 44% and a specificity of 95% at the cut-off of 0.08.