To evaluate the prognostic significance of post-irradiation biopsy results in patients with prostatic cancer, we reviewed the records of 803 patients who had been treated with pelvic lymph node dissection, radioactive gold seed implantation and external beam irradiation. Of the patients 124 had 1 or more biopsies within 6 to 36 months after completion of radiotherapy when there was no evidence of local or distant recurrence of tumor. Patients were followed for a mean of 64 months (range 14 to 175 months) and received no other therapy before relapse. Over-all, 43 of these patients (35 per cent) had a positive biopsy result. The incidence of positive biopsy results correlated directly with the initial stage of the tumor, ranging from 22 per cent of stage B1N to 50 per cent of stage C1 lesions. However, biopsy results did not correlate with the grade of the tumor. Local recurrence and distant metastases were much more common among patients with a positive biopsy result (p equals 0.0006). Local recurrence developed in 58 per cent of the patients with a positive biopsy by 5 years and in 82 per cent by 10 years. Of those in whom all biopsies were negative only 18 per cent had local recurrence by 5 years and 32 per cent by 10 years. Biopsy results retained their prognostic significance even among the more favorable subset of patients whose pelvic lymph nodes were negative initially and those with a normal prostatic examination at biopsy. These results indicate that a post-irradiation prostate biopsy 6 to 36 months after completion of treatment can be used to determine the efficacy of a particular radiotherapeutic regimen as well as the success or failure of radiotherapy in an individual patient.