We undertook a study over a period of 9 months to define the frequency of parasitic infections in hospitalized children with diarrhoea between the ages of 15 months and 5 years. Every alternate day, mothers of all children admitted with diarrhoea between 09.00 hours and 12.00 hours to one of the wards of the Department of Pediatrics and Child Health of the University Teaching Hospital (UTH) in Lusaka, Zambia, were interviewed for enrollment of their children into the study. A total of 178 children with diarrhoea were enrolled in the study. Of these 44 (25 per cent) were HIV seropositive and 134 (75 per cent) were seronegative for HIV. Out of 44 HIV-seropositive patients, 20 (45 per cent) had acute diarrhoea and 24 (55 per cent) had chronic diarrhoea. Of the 134 HIV-seronegative patients, 68 had acute diarrhoea (51 per cent) and 66 (49 per cent) had chronic diarrhoea. At least one intestinal parasite was found in 34 out of the 178 children enrolled. The commonest parasites identified were Ascaris and Cryptosporidia. No associations were identified between parasite isolation and the following: age, sex, or socio-economic status. Cryptosporidium spp. was isolated from 6 out of 44 (14 per cent) HIV-seropositive children, while 8 out of 134 (6 per cent) seronegative children had the parasite (P = 0.01). HIV-seropositive children with chronic diarrhoea had significantly higher cryptosporidium identification rates than those HIV-seropositive children with acute diarrhoea [5 out of 24 (21 per cent) patients with chronic diarrhoea compared to 1 out of 20 (5 per cent) patients with acute diarrhoea; (P > or = 0.01)].(ABSTRACT TRUNCATED AT 250 WORDS)