From 1968 to 1975 105 patients with adenocarcinoma of the endometrium, FIGO clinical Stage I, were randomly allocated to receive, prior to hysterectomy, either a single implant with Heyman capsules and/ or tandem and ovoids, or external megavoltage irradiation. There were no significant differences between the two study arms with respect to distribution of age, uterine size, obesity, frequency of diverticular disease, or histologic grade. Complications were graded rigorously to assure recording all possible treatment related complications and to minimize under-reporting of complications not obviously or directly attributable to the radiation. Fifty-five patients received intracavity irradiation and experienced 5 - and 10-year actuarial disease-free survivals of 80% and 67%, respectively, as compared to 70% and 59% for 50 patients who received external beam. There were only 4 recurrences in the intracavitary group versus 14 in the external beam group. One half of the recurrences in each group were in the pelvis. Major complications occurred with equal frequency in both groups, but minor complications were much more frequent in the external beam group. The differences in survival, recurrences and minor complications were statistically significant, with P values of 0.023, 0.03, and less than 0.02, respectively. With the techniques utilized here, intracavitary radiation is thus seen to be superior to external beam irradiation in terms of higher disease-free survival, lower frequency of recurrence and fewer complications.