[en] Objectives: Prostate-specific antigen (PSA) test has become one of the most cost effective tools for detecting early prostate cancer. In general, a diagnosis of prostate cancer is uncommon at serum levels of total PSA (TPSA) at or below 4 ng/mL and is common at levels above 10 ng/mL. The diagnostic gray zone is between 4.0 and 10.0 ng/mL, where the differential diagnosis of prostate cancer is most difficult. For such patients ratio of free-to-total PSA (F/TPSA) can be useful in differentiating prostate cancer from benign prostate hypertrophy (BPH). However, screen for prostate cancer with PSA remains controversial. Thirty-five percent of the patients with clinically localized prostate cancer present with serum PSA levels below 4 ng/mL. What is the optimal 'reflex' total PSA at which we should implement use of the F/T PSA? The purpose of this study is to assess the usefulness of F/TPSA in patients with TPSA less than 4 ng/mL. Methods: A total of 101 high-risk patients of prostate cancer underwent transrectal ultrasonography and biopsy or transurethral resection of prostate were studied. Sixty-eight patients were proved to have BPH only and 33 patients were proved to have prostate cancer. TPSA and F/TPSA were determined using a immunoradiometric assay (PSA-RIACT, FPSA-RIACT, CIS). The appropriate cut-off value of F/TPSA in diagnosis of prostate cancer determined by receiver-operating characteristic (ROC) curve was 0.19. Results: The TPSA value of the subjects ranged from 0 to 15 ng/mL. Seventeen cases (26%) of prostate cancer were disclosed in 65 patients with TPSA > 4 ng/mL. Thirty-six patients had TPSA < 4 ng/mL were analysed. Among them 17 patients (47%) had F/T PSA < 0.19, and 14 of them (39%) were proved to have prostate cancer. Only 2 patients with TPSA < 4 and F/T PSA > 0.19 were proved to have prostate cancer. Conclusion: Thirty-nine per cent of high-risk patients with TPSA < 4.0 ng/mL and F/T PSA < 0.19 was found to have prostate cancer. F/T PSA should be determined in Chinese patients with TPSA < 10 ng/mL instead of the algorithm of combined use of F/T PSA and TPSA between 4-10 ng/mL