In a prospective clinical trial 143 women undergoing pelvic radiotherapy for gynaecological malignancies, were randomized to receive either a low-fat, low-lactose diet (intervention group) or a regular diet (control group) in order to evaluate the possible impact of diet therapy on radiation induced diarrhoea, nausea and vomiting. The daily number and consistency of stools, use of antidiarrhoeal agents, nausea and vomiting were recorded before radiotherapy was begun (week 0), in the last week of therapy (week 6) and 6 weeks after the end of therapy (week 12). The intervention group used half the amount of antidiarrhoeal agents in week 6, than used by the control group (mean 0.6 tablets per day versus 1.1, p < 0.01). 14 patients (23%) in the intervention group reported diarrhoea, versus 32 (48%) in the control group (p < 0.01). In week 12 there were no differences in the use of antidiarrhoeal agents and the prevalence of diarrhoea between the groups.