The best available data indicate that, although it is imperfect, the postirradiation biopsy performed at a sufficient interval after radiotherapy can provide accurate prognostic information useful in the determination of the success or failure of radiotherapy in an individual patient as well as the measurement of overall efficacy of any particular radiotherapeutic regimen. Needle biopsy of the prostate was performed routinely in 510 patients with clinical stage A2, B, or C1 prostate cancer treated with a combination of radioactive gold seed implantation and external-beam irradiation. Of the 140 patients who had one or more needle biopsies performed 6-36 months after completion of radiotherapy, who had no evidence of local recurrence or distant metastases at the time of biopsy, and who had received no hormonal therapy before documented recurrence of the tumor, 45 (32%) had one or more biopsies positive for cancer. The frequency of positive biopsy results correlated significantly with the size of the local tumor but not with the grade. The correlation between biopsy results and the eventual development of recurrence was highly significant. If any biopsy was positive, 60% of the patients eventually developed local recurrence; if all biopsies were negative, only 19% developed local recurrence during the period of follow-up. The poor prognosis associated with a positive biopsy result was found within almost every subset of stage, grade, or nodal status examined although the results varied because of the small number of patients in some groups.