Infectious human immunodeficiency virus (HIV) was recovered from two out of four bowel biopsy specimens from acquired immunodeficiency syndrome (AIDS) patients with chronic diarrhoea of unknown aetiology. In-situ hybridisation of biopsy specimens from rectum and duodenum of other AIDS patients with gastrointestinal complaints showed the presence of HIV-infected cells in both the base of the bowel crypts and the lamina propria. The type(s) of epithelial cell(s) infected could not be determined definitively. However, the association of in-situ labelling of HIV RNA in argentaffin staining cells strongly suggests that enterochromaffin cells derived from neural crest tissue are among the target cells. This evidence that HIV can directly infect the bowel raises the possibility that the virus causes some of the gastrointestinal disorders of AIDS patients.