The effects of the volume of irradiated small bowel on late small-bowel tolerance was studied, taking into account the equivalent total dose and type of pre-irradiation surgical procedure. A method was developed to estimate small-bowel volumes in the high-dose region of the radiation treatment using CT-scans in the treatment position. Using this method small-bowel volumes were measured for three-field and AP-PA pelvic treatments (165 cm3 and 400 cm3, respectively), extended AP-PA pelvic treatment (790 cm3), AP-PA treatment of para-aortic nodes (550 cm3) and AP-PA treatment of para-aortic and iliac nodes (1000 cm3). In a retrospective study of 111 patients irradiated after surgery for rectal or recto-sigmoid cancer to a dose of 45-50 Gy in 5 weeks, extended AP-PA pelvic treatment (n = 27) resulted in a high incidence of severe small-bowel complications (37%), whereas for limited (three-field) pelvic treatment (n = 84) the complication rate was 6%. These complication data together with data from the literature on postoperative radiation-related small-bowel complications were analysed using the maximum likelihood method to fit the data to the logistic form of the dose-response relation, taking the volume effect into account by a power law. The analysis indicated that the incidence of radiation-related small-bowel complications was higher after rectal surgery than after other types of surgery, which might be explained by the development of more adhesions. For both types of surgery a volume exponent of the power-law of 0.26 +/- 0.05 was established. This means that if the small-bowel volume is increased by a factor of 2, the total dose has to be reduced by 17% for the same incidence of small-bowel complications.