Serial serum prostate-specific antigen (PSA) levels were analyzed retrospectively for prognostic implications in 70 patients with locoregional (Stages B2, C, and D1) prostate cancer who were managed with high energy neutron beam therapy. Three groups of patients were identified. Group I included 30 patients whose serum PSA level decreased to the reference range (0-4 ng/mL) following neutron therapy and remained so subsequently: 28 (93%) remained disease-free and 2 (7%) have failed distantly. All 30 patients (100%) had no evidence of locally progressive disease. This group was categorized as having a good prognosis. The mean time for serum PSA value to decline to reference range was six months; calculated mean time to achieve a stable base-line PSA was 53 +/- 37 days. Follow-up period ranged from twelve to fifty-six months (median: 21 months). Group II consisted of 13 patients in whom there was an initial decrease in serum PSA to reference range followed by a subsequent increase: 6 of 13 (46%) have no overt clinical progression of disease; 7 (54%) have either persistent locoregional or distant metastatic disease. Follow-up period was from twelve to seventy-two months (median: 39 months). Calculated mean time to achieve stable baseline PSA for serum PSA in this group was 61 +/- 21 days. Group III patients had a persistently elevated or rising serum PSA concentration. Of 27 patients in this group, only 9 (33%) have no evidence of disease progression, while 18 patients (67%) have failed already, either locoregionally or distantly. Follow-up period ranged from twelve to sixty-nine months (median: 21 months). Mean time to achieve stable baseline of serum PSA in this cohort of patients with a poor prognosis was 108 +/- 76 days. We conclude that PSA has a predictable prognostic value in patients with locally advanced prostate cancer managed with high energy neutron beam therapy. Rapid normalization of PSA after therapy indicates a good prognosis. Persistent elevation signifies either presence of persistent locoregional disease or development of distant metastases. Subsequent elevation of the serum PSA concentration after definitive therapy signals progression of prostate cancer.