Previous studies using dual sugar permeability tests suggested that damage to the small intestinal mucosa plays an important part in the development of persistent diarrhea in The Gambia. The present study has extended these findings by examining the effect of nutritional rehabilitation on intestinal permeability and mucosal morphology. Intestinal permeability, measured by lactulose:mannitol (L:M) absorption, and mucosal structure, measured by a quantitative, computerised morphologic technique, were evaluated in 20 children before and after such treatment. L:M ratios were high on admission, (0.66 +/- 0.36) and, despite some temporary improvement, did not significantly improve (0.49 +/- 0.30) following rehabilitation for one month. The changes in L:M ratio were largely due to an increase in lactulose absorption, showing that the small intestinal mucosa becomes more "leaky" as a result of nutritional rehabilitation. Although no correlation was found between measures of intestinal permeability and mucosal morphology, nutritional restitution was associated with a significant increase in size of the mucosal crypt cell compartment, but not in villous epithelial volumes during the same period. It is necessary to establish, by further prospective studies, the interval required for full restitution of small intestinal structure and function during treatment for persistent diarrhea.