A series of 83 rectal biopsies was examined by electron microscopy (EM), 78 from HIV-seropositive patients (64 with diarrhoea, 9 with weight loss, and 5 asymptomatic) and 5 from controls without risk factors for HIV infection. Although EM identified the likely cause of the diarrhoea in only nine cases, the majority of symptomatic HIV-seropositive patients had abnormal biopsies including the presence of tubuloreticular structures, loss of microvilli, and oedema and disruption of autonomic nerves. Neuronal abnormalities were particularly common in patients with cryptosporidiosis, but no ultrastructural marker was specific to 'pathogen-negative' diarrhoea.