Malignant peritoneal cytology in patients with endometrial carcinoma is a poor prognostic feature, identifying patients at high risk for early intra-abdominal recurrence. Between 1977 and January, 1983, 65 women with endometrial carcinoma who had malignant peritoneal cytology were treated with adjuvant intraperitoneal radioactive chromic phosphate P 32 suspension. Fifty-three patients (80%) were clinical Stage I, nine (14%) were Stage II, and three (7%) were clinical Stage III. Life-table estimates of disease-free survival were 89% for clinical Stage I patients and 94% for surgical Stage I patients beyond 24 months. One patient developed an intraperitoneal recurrence, four had simultaneous intraperitoneal and extraperitoneal recurrences, and six developed recurrences outside of the peritoneal cavity. Few significant acute complications occurred after therapy with radioactive chromic phosphate P 32 suspension. Chronic intestinal morbidity that required surgical correction was encountered in five of 17 patients (29%) who received adjuvant pelvic radiation, compared to none of the 48 patients (0%) who received only radioactive chromic phosphate P 32 suspension (p less than 0.001). Intraperitoneal instillation of radioactive chromic phosphate P 32 suspension is effective therapy for patients with malignant peritoneal cytology from endometrial carcinoma. Caution should be exercised when radioactive chromic phosphate P 32 suspension and external radiation therapy are combined.