A method is presented of calculating biological doses in combined external and intracavitary radiotherapy with long-lived radioisotopes. The method utilizes the well-known "nominal standard dose" concept combined with an original correction, based on a radiobiological model, to account for the nonhomogeneity of the external field therapy. Of practical interest are the points of maximal radiation dose sustained by the rectum and bladder in the treatment of carcinoma of the uterine cervix. Forty patients with malignant tumors of the uterine cervix who were treated at the Beilinson Hospital are described. A high degree of correlation was found between the biological dose and long-term regional complications. There appears to be a practical threshold dose, of about 4,000 rets, below which severe gastrointestinal and genitourinary complications are rare. A strategy of optimal treatment leading to limited complications is presented.