Twenty-five patients with abdominopelvic malignant tumors receiving irradiation were studied. The following items were assessed before, during, and at the end of the radiation period: 1) clinical symptoms, scored on the basis of their severity; 2) orocecal transit time (lactulose-H2 breath test); and 3) absorption of lactose and bile salts. Ninety-two percent of the patients showed clinical symptoms suggestive of acute radiation enteropathy, mainly diarrhea. A significant acceleration of the orocecal transit was observed during the study: 70 +/- 6.9 min vs 63.3 +/- 6.3 min vs 44.3 +/- 5.1 min (F = 5.49, p = 0.008), being faster in patients with severe diarrhea (F = 3.25, p = 0.05). Forty-four percent and 57% of the patients developed lactose and bile salt malabsorption, respectively. A decrease in the transit time was observed, independent of the absorption or malabsorption of such substances. However, the orocecal transit was faster in those lactose malabsorbers with severe diarrhea, than in those with mild diarrhea (F = 4.2, p = 0.03). The results suggest that acceleration of orocecal transit may be a major factor in the pathophysiology of radiation-induced diarrhea, whereas lactose malabsorption may contribute to the severity of the diarrhea.