Impaired gastrointestinal function was detected during the acute phase of illness in patients with Plasmodium falciparum malaria. Nine of 11 (82%) had diminished D-xylose absorption, 6 of 10 (60%) had decreased absorption of vitamin B12, and 5 of 11 (45%) had low serum carotene levels. A majority of the tests for absorptive function returned toward normal during the convalescent period. Jejunal biopsies in 6 of 7 patients with malaria revealed edema and round cell infiltration of the lamina propria, and shortening and widening of villi. In 5 of these biopsies the presence of malarial pigment and parasitized red blood cells was noted within the mucosal vessels. In each case, the morphologic changes reverted toward normal during convalescence. The pathophysiologic mechanism responsible for the malabsorption syndrome in malaria has not been elucidated but may be due in part to ischemic changes in the mucosa of the small bowel.