Twelve of 304 patients (4%) treated with surgery and adjuvant irradiation for endometrial carcinoma experienced a serious complication. The complication rate for patients whose irradiation consisted of an implant alone was only 1% (two of 199), if the implant was preoperative, but was 12% (three of 26) if the implant was postoperative. For patients who received external pelvic irradiation as part of their adjuvant therapy, the incidence of complications was 8.8% (seven of 79) and the timing (preoperative versus postoperative) had no effect. There was, however, a significant correlation of the complication rate with an increasing dose of external irradiation to the whole pelvis: For doses of 3000 rad or less, it was 2% (five of 264) but was 18% (seven of 40) for doses in excess of 3000 rad. These data suggest that the safest form of adjuvant irradiation for adenocarcinoma of the endometrium is a preoperative implant, and that adding external pelvic irradiation to the preoperative implant will significantly increase the complication rate if the external dose to the central pelvis exceeds 3000 rad.