The clinical significance of serum prostate specific antigen after primary irradiation for adenocarcinoma of the prostate is uncertain. Between September 1986 and December 1987 serial prostate specific antigen values were determined in 43 patients before and after definitive radiation therapy. The study group included 6 patients with stage T0d, 10 with stage T1, 11 with stage T2 and 16 with stage T3 disease, with a mean pre-treatment prostate specific antigen level of 49.2 +/- 10.8. For all patients the first post-treatment prostate specific antigen level was less than the pre-treatment level. One patient failed locally with recurrent prostatic cancer that invaded the rectum. The 6 patients who failed with symptomatic metastases had an increasing prostate specific antigen level 2 to 7 months before detection of recurrence. Based on the absolute value and trend of the prostate specific antigen, patients were described as being at high, intermediate or low risk for distant metastases. Of 9 high, 6 intermediate and 28 low risk patients 4 (44%), 2 (33%) and 0 (0%) have experienced recurrent disease (p = 0.0025). We conclude that serial post-irradiation prostate specific antigen values may be useful in the early identification of patients at risk for treatment failure.