THE RADIATION THERAPY ONCOLOGY GROUP (RTOG) has conducted two studies testing the value of extended-versus limited-fields in carcinoma of the prostate. RTOG protocol 75–06 tests the value of paraaortic irradiation in patients with locally advanced tumors limited to the pelvis. RTOG protocol 77–06 tests the value of pelvic irradiation in patients with tumors limited to the prostate. Two hundred sixty-eight evaluable patients have been entered on RTOG 75–06, and 219 on RTOG 77–06 prior to 1981. This is a preliminary report on treatment-related morbidity in patients followed for a minimum of 1 year. Treatment-related morbidity has been classified according to a five-grade severity classification schema and correlatet with the protocol option assignment in order to determine whether the use of extended fields had an effect on the incidence of side effects. Thirty-one percent of 268 patients on RTOG 75–06 and 34% of 219 patients on RTOG 77–06 had some form of treatment side effects. The majority (over 80%) of these were mild (grade 1 and 2) and by definition did not interfere with the patients' performance (life style). Only three patients on RTOG 75–06 and one on RTOG 77–06 had grade 4 complications (by definition requiring a surgical intervention). No fatal (grade 5) complications have been recorded so far. The use of paraaortic fields in RTOG 75–06 and pelvic fields in RTOG 77–06 have not resulted in a significant increase of GI or GU morbidity at this time. The only statistically significant trend was the incidence of postirradiation genital and lower extremity edema which strongly correlated with the extent of staging lymphadenectomy.