In a prospective clinical and bacteriological study the endocervical and endometrial flora of 38 patients with biopsy proven cancer of the endometrium was studied before and after intrauterine irradiation with 10 Gy (Ir-192, afterloading device, Buchler, Braunschweig). Bacteriological swabs were taken transcervically. Anaerobic transport media were used and cultures were done aerobically and anaerobically. The mean number of aerobic organisms per patient increased from 2.13 to 2.49 (not significant (n.s.)), and of anerobic organisms from 1.42 to 2.00 (n.s). Neither the recovery rate of aerobic and anaerobic bacteria nor the composition of endometrial flora was altered significantly by intracavitary irradiation. Only aerobic sporeformers increased from 5.3% to 21% (2 p less than 0.05). The most prevalent aerobic species were S. epidermidis, streptococci and E. coli, the predominant anaerobic species Pepto- and Peptostreptococcus and Bacteroides. Our results clearly show that the intrauterine administration of 10 Gy by a Ir-192-afterloading device does not sterilize the endocervix or endometrium and infections may occur after intracavitary radiotherapy.