From 1971 to 1981, 26 patients with pathologic Stage C prostate cancer were referred to this department for postoperative radiation therapy. During the same period 24 patients, undergoing radical prostatectomy at this institution, were found to have pathologic Stage C disease but did not receive adjuvant irradiation. Both groups of patients are compared at 5 years post surgery for disease-free survival, local control, and distant failure in an attempt to determine whether there is a demonstrable benefit of adjuvant irradiation in this setting. All irradiated patients started therapy 1 to 4 months post surgery. All were treated with megavoltage irradiation. The majority were treated to the pelvis by a 4-field technique to an absorbed dose of 46-50 Gy followed by a prostate boost via multiple fields to a total dose of 54-70 Gy. Three patients received 50-52 Gy to the pelvis only and 5 patients received 56-65 Gy to localized prostate fields only. Moderate complications in the irradiated group were 23% with no severe complications observed. This rate may be reduced with treatment of local prostate fields only. There was no significant difference in 5-year survival, 5-year disease-free survival, or distant metastases in patients treated with surgery only or surgery plus radiation. However, local recurrence at 5 years was significantly different (p = 0.046), 16.6% in the surgery only group and 0 in the irradiated group. This difference was more pronounced in the subset of patients with seminal vesicle invasion: 30% local recurrence in the surgery only group versus 0 in the irradiated group. These data suggest that the addition of postoperative irradiation in pathologic Stage C prostate cancer yields an improvement in local control, and that for the subset of patients at significant risk of local relapse, those with seminal vesicle invasion, adjuvant prostate bed irradiation should be recommended.