Diarrhoea is common in patients infected with the human immunodeficiency virus (HIV). We sought the cause of diarrhoea in all HIV antibody-positive patients with diarrhoea who presented at St Stephen's Hospital, London over a period of 15 months. Altogether, 441 specimens from 179 patients were examined. Infective causes were found in 86 (48 %) patients. Protozoa were the most common infecting organisms (30% patients). Of these, Cryptosporidium sp. was the most frequent (9·5% patients), followed by Entamoeba histolytica and Giarda lamblia. 'Non-pathogenic' protozoa (NPP) were also common (15 % patients), often in the absence of generally recognised pathogens. A case controlled study failed to show a significant difference in the rate of isolation of NPP in HIV antibody-positive patients with diarrhoea compared with HIV antibody-positive patients without diarrhoea. Bacterial causes of diarrhoea were found as follows: Salmonella spp., Campylobacter spp. and Shigella sonnei; Mycobacterium avium-intracellulare (MAI) was isolated from the faeces of eight (4·5%) patients. Isolation of MAI from faeces was associated with disseminated MAI infection. This study has shown that two commonly isolated pathogens, namely Cryptosporidium sp. and MAI, can be identified quickly and reliably by the same modified Ziehl-Neelson staining of concentrated faeces.