Serum prostate specific antigen was determined (Yang polyclonal radioimmunoassay) in 183 men after radiation therapy for adenocarcinoma of the prostate. A total of 163 men had received 7,000 rad external beam radiotherapy and 20 had been implanted with 125iodine seeds. Only 11 per cent of these 183 patients had undetectable prostate specific antigen levels at a mean interval of 5 years since completion of radiotherapy. Prostate specific antigen levels after radiotherapy were directly related to initial clinical stage and Gleason score before treatment. Multiple prostate specific antigen determinations were performed with time in 124 of 183 patients. During year 1 after radiotherapy prostate specific antigen levels were decreasing in 82 per cent of the patients but only 8 per cent continued to decrease beyond year 1. Of 80 patients observed greater than 1 year after completion of radiotherapy 51 per cent had increasing values and 41 per cent had stable values. Increasing prostate specific antigen values after radiotherapy were correlated with progression to metastastic disease and residual cancer on prostate biopsy. Total serum acid phosphatase levels were poorly related to prostate specific antigen levels, were less effective in discriminating patients with metastatic disease and provided no additional information beyond that provided by prostate specific antigen.