For this study, 136 patients treated at Stanford University Hospital for prostatic cancer between 1971 and 1980 were selected for review. The patients had received no prior therapy, and had no evidence of bone metastases at time of radiation treatment based on radiographic studies and bone scan. Of this group, 71 patients received extended-field irradiation (paraaortic and pelvic fields), and 65 patients received pelvic irradiation. The pelvic field was treated to 50 Gy and the paraaortic field received 45 Gy to 60 Gy. All patients subsequently underwent routine follow-up examinations and studies at Stanford University Hospital: 1,513 follow-up X rays, bone scans, and CT-scans were analyzed for site-specific recurrence. The follow-up ranged from 14 months to 16 yrs from the time of initial treatment, with a mean follow-up of 7 yrs. Lower extremities and ribs were found to be the most common sites of bone metastases. Irradiation of the lumbar spine to a dose of 35 to 60 Gy, coincidental to irradiation of the paraaortic lymph nodes prevented or delayed the development of lumbar spine metastases. The potential mechanism and clinical implications are discussed.