Diarrhoea and weight loss are common features of human immunodeficiency virus (HIV) disease. The mechanism of diarrhoea occurring in the absence of known enteropathogens is currently unknown. We have measured fat absorption, using the 14C triolein breath test, and quantitatively assessed jejunal villous architecture in 20 male homosexuals at various clinical stages of HIV disease. Enteropathogens were not detected in any subject at the time of jejunal biopsy in stool or jejunal mucosa. Partial villous atrophy was the sole histological abnormality and was detected at any clinical stage of HIV disease. The 14C triolein breath test quantitatively correlated with the degree of jejunal villous atrophy. In addition subjective presence of diarrhoea was related to the detection of fat malabsorption. Thus diarrhoeal disease in HIV infected patients in the absence of enteropathogens may be due to jejunal enteropathy and may be present at early clinical stages of HIV disease.