To evaluate the long-term effects of abdominal irradiation for the treatment of seminoma of the testis on gastrointestinal function, 15 randomly selected patients who had been treated for stage I seminoma of the testis 2-10 years previously had the following parameters of gastrointestinal function evaluated: gastrointestinal symptoms; absorption of bile acid; vitamin B12; lactose and fat; gastric emptying; small intestinal and total gut transit; and intestinal permeability. Results were compared to those obtained in 18 normal volunteers. There was an increased prevalence of gastrointestinal symptoms (P < 0.01) in the patients and stool frequency was above the control range in two of them. Gastric emptying was faster (P < 0.01) in the patients. There were no significant differences in vitamin B12, bile acid, lactose or fat absorption, small intestinal transit or whole gut transit between the two groups, although faecal fat excretion was greater than the control range in three of the patients. At least one parameter of gastrointestinal function was abnormal in 11 of the 15 patients. Patients with right-sided seminoma had a greater bowel frequency when compared to those with left-sided seminoma (P < 0.05). We conclude that mild abnormalities in gastrointestinal function occur frequently when abdominal irradiation is used to treat stage I seminoma.