OBJECTIVE We determine which clinical characteristics correlate with a successful outcome following external beam radiation for the management of biopsy proved, locally recurrent prostate cancer after radical prostatectomy. METHODS The clinical records of 34 patients who were treated at our institution with external beam radiation for biopsy proved local disease recurrence after radical prostatectomy were reviewed. Mean followup was 77.9 months after radical prostatectomy and 38.3 months after radiotherapy. Preoperative, postoperative and pathological characteristics were examined for the ability to predict failure following radiotherapy. RESULTS Of the 34 patients 9 (26%) exhibited persistently low and stable serum prostate specific antigen (PSA) (less than 0.5 ng./ml.), while 7 (21%) had an undetectable serum PSA (less than 0.1 ng./ml.) during followup. These 16 patients were considered to be successfully treated by radiation and in the remaining 18 (53%) radiation was considered to have failed. The likelihood of successful treatment at 3 years after radiotherapy for all patients was 48%. Preoperative PSA, PSA at first elevation, postoperative PSA velocity and pathological stage were not significant predictors of a successful outcome following radiation treatment. Patients with a serum PSA of 4 ng./ml. or less before receiving radiotherapy and those with a prostatectomy specimen Gleason score of 7 or less were significantly more likely to be successfully treated by radiotherapy. CONCLUSIONS Radiation therapy is a viable treatment option for select patients with biopsy proved local disease recurrence following radical prostatectomy. The chance of achieving and maintaining a persistently low and stable or undetectable serum PSA is likely in those patients with a prostatectomy specimen Gleason score of 7 or less and a pre-radiation PSA of 4.0 ng./ml. or less. More effective treatment regimens are needed for those patients in whom radical prostatectomy fails.