The presence of gas in bowel could lead to higher than expected doses being given in the radiotherapy of abdominal and pelvic tumors. Radiation detectors were placed in the uterus and vagina of ten patients with caricnoma of cervix, and the actual dose compared with that expected when anterior fields of treatment were employed. Elvations greater than 10% were obtained in six, and severe postradiation morbidity subsequently occured in two of the three showing the highest readings.