Prostate specific antigen (PSA) and gamma-seminoprotein (gamma-Sm) are used as tumor markers of the prostate cancer. However, the serum concentrations of PSA and gamma-Sm are frequently increased in patients with benign prostatic hypertrophy (BPH). We measured the ratio of serum PSA to gamma-Sm concentration (P/S ratio), and evaluated its usefulness for diagnosis of prostate cancers. Between April 1988 and July 1992, 162 men underwent prostatic biopsy and/or TUR-P, and were diagnosed pathologically. Of 162 patients, 112 were diagnosed as BPH and 50 were diagnosed as prostate cancer. Of 24 patients with serum PSA level of > 20 ng/ml, 23 (95.8%) were prostate cancer, while, of 79 patients with serum PSA level of 3.0-20 ng/ml, 23 (29.1%) were prostate cancer. The sensitivity and the specificity for PSA were 92.0% and 49.1%, respectively. Of 85 patients with serum gamma-Sm level of > 4.0 ng/ml, 30 (35.3%) were prostate cancer. The sensitivity and the specificity for gamma-Sm were 60.0% and 50.9%, respectively. A mean +/- SD of P/S ratio in 112 patients with BPH was 0.954 +/- 0.591. While, the mean +/- SD of P/S ratio was 16.295 +/- 58.584 in all prostate cancer patients, and 2.031 +/- 0.654 in 27 prostate cancer patients with serum PSA level of < or = 20 ng/ml. P/S Ratio in prostate cancer patients with serum PSA of < or = 20 ng/ml as well as in all prostate cancer patients were significantly higher than P/S Ratio of BPH patients (p < 0.0001). Of 55 patients with P/S Ratio of > or = 1.50, 45 (81.8%) were prostate cancer and 10 (18.2%) were BPH. While, of 107 patients with P/S Ratio of < 1.50, 102 (95.3%) were BPH and 5 (4.7%) were prostate cancer. The sensitivity and the specificity for P/S Ratio were 90.0% and 91.1%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)