To evaluate the usefulness of tumor markers in monitoring the patients with prostate cancer, serial measurements of serum prostate-specific antigen (PA), prostatic acid phosphatase (PAP) and gamma-seminoprotein (gamma-Sm) were performed in 78 stage C or D patients. Positive rates of each marker prior to the treatment were as follows; PA; 75%, PAP; 56% and gamma-Sm; 62% in stage C, and PA; 95%, PAP; 79% and gamma-Sm; 91% in stage D. In most cases showing PR (partial response) and S (stable) in clinical responses, these three markers decreased their serum titers corresponding to clinical course if the markers were elevated at the start of the treatment. But the usefulness of PAP was lessened because of its lower positive rate than those of PA and gamma-Sm. In 33 PD (progressive disease) cases, positive rates of each marker at time of clinical diagnosis as PD were found to be 85% in PA, 55% in PAP and 76% in gamma-Sm. And with the combination assays of these three tumor markers, positive rate was elevated to 88%. Moreover, elevation of serum values of these three markers at 3 months before the progression event were observed in 50% of PA, 39% of PAP and 46% of gamma-Sm. Then the prognostic significance of each marker was examined. In PA and PAP, there were statistical differences in non-relapsing rates between patients whose reduction rates from the pretreatment value on 7th day were more than and less than 50%. But in gamma-Sm, a statistical difference between each group was firstly observed on 14th day. As a result, in monitoring patients with prostate cancer, PA and gamma-Sm are more useful than PAP and, in prediction of patients' prognosis, PA is more useful than gamma-Sm.