In a prospective bacteriological and clinical study the vaginal bacterial flora of 35 patients with endometrial carcinoma, who underwent surgery 4 to 6 weeks earlier, was investigated prior to and following intracavitary vaginal irradiation with 10 Gy in 0,75 cm(Ir-192, afterloading device, Buchler). Bacteriological swabs were taken prior to and following ther insertion of a tube applicator. Anaerobic transport-media were used an cultures were performed aerobically and anaerobically. The mean number of aerobic species per patient increased slightly from 3.26 to 3.60 (n.s), anaerobic species remained constant (1.36 before and 1.30 after irradiation). Also the frequency of isolation of most aerobic and of all anaerobic species was statistically not altered following irradiation. Staphylococcus epidermidis decreased from 51.4% to 22.8% significantly (2 p less than 0.05), Micrococci increased from 0.0% to 11.4% (2 p less than 0.05) and aerobic sporeformers form 0.0% to 34.3% (2 p less than 0.01). Among the anaerobes Pepto- and Peptostreptococci were isolated prior to and following intracavitary irradiation in 11 to 14%, Bacteroides-species in 11 to 20%. Our bacteriological results suggest, that intracavitary irradiation in the therapeutically usual doses has no sterilizing effect on the vaginal flora. This flora plays an important role in infection following gynecologic radiotherapy.