1. Small-intestine integrity in Caucasian and African patients infected with human immunodeficiency virus was determined by measuring the permeation across the mucosa of two sugars, lactulose and mannitol. 2. The sugars were assayed by h.p.l.c. and pulsed amperometric detection in 6 h urine samples. Stool microscopy for enteropathogens was performed in all patients. 3. The ratio of lactulose to mannitol recovered in urine was increased in Caucasian and African patients with advanced human immunodeficiency virus infection. Asymptomatic human-immunodeficiency-virus-infected subjects had a normal lactulose/mannitol ratio. African patients with diarrhoea showed a twofold reduction in mannitol excretion. Such a change in mannitol absorption was not detected in Caucasian patients and occurred regardless of the presence of enteropathogens. 4. Altered small-intestinal permeability is associated with symptomatic diarrhoea in human immunodeficiency virus infection in both Caucasian and African patients.