Small intestinal absorptive function was investigated in six patients with the acquired immunodeficiency syndrome (AIDS) who had diarrhoea and weight loss. Proximal function was assessed by [14C]Triolein test of fat absorption. Distal function was determined by a test of bile acid absorption in which the loss of radio-labelled synthetic bile acid, 75seleno-23-homocholic acid-taurine ([75Se]HCAT), from the enterohepatic circulation was quantified by abdominal gamma-scanning and by a vitamin B12-intrinsic factor absorption test. Concurrently indirect tests of small intestinal bacterial overgrowth ([14C]glycocholate and breath hydrogen) were carried out. In addition, jejunal histological examination and stool microscopy and culture for enteropathogens were performed. Fat absorption was reduced in all six patients, four of whom had jejunal villous atrophy. Bile acid and vitamin B12 absorption were normal in four subjects. Enteropathogens were not detected in any of the four subjects with normal terminal ileal absorptive function. In contrast, reduced bile acid and vitamin B12 absorption were detected in two of six subjects. Both patients had an enteropathogen (Cryptosporidium spp. and Isospora belli) present on stool and jejunal histological examination. Neither subject had evidence of small-intestinal bacterial overgrowth. AIDS patients therefore may have normal ileal absorptive function in the presence of jejunal disease. Infection with Cryptosporidium spp. or I. belli may however, be associated with severe ileal dysfunction.