The significance of prostatic specific antigen (PSA) was investigated in the subjects examined by the mass screening for prostate cancer from 1985 to 1990. All subjects was examined by digital rectal examination (DRE) and with prostatic acid phosphatase (PAP) and the subjects in whom prostate cancer (Pca) was suspected from abnormal DRE and/or elevated PAP were recommended to receive the secondary screening to confirm the presence of Pca. PSA was measured by radioimmunoassay using Ball-Elsa-PSA-kit. 1,600 serum samples were obtained from our serum bank. The relationship among PSA, prostate size estimated by DRE and age was investigated. PSA was increased with age and the prostate size, PSA being more closely related with the latter. Therefore, we estimated that PSA has an ability to detect benign prostatic hypertrophy (BPH) in the mass screening. This estimation should be confirmed by using an ultrasound tomography because the prostate size obtained by DRE is inaccurate as compared with that obtained by ultrasound tomography. The cut off level of PSA was determined by control which was composed from the subjects with normal size prostate and one with BPH. When the cut off level was 8.6 ng/ml, the sensitivity, specificity and efficiency as Pca marker was 73.9%, 97.4% and 97.1%, respectively. PSA was more than 8.6 ng/ml in all of Pca with elevated PAP. PSA was expected to improve the Pca detection rate in our mass screening system.