The investigation of an outbreak of cryptosporidiosis in a hospital day-care centre is reported. Twenty-eight (27%) children and one member of the staff were infected. Most of the cases were symptomatic and the major presenting symptom was watery diarrhoea. Shedding of oocysts continued for a mean of 12 days after diarrhoea had subsided and most of the cases were still excreting oocysts when the first follow-up sample was examined. Intermittent shedding or a carrier state were not seen in any of the cases. Giardia intestinalis cysts were detected in 19 children, 7 of whom showing mixed infection with Cryptosporidium sp., but this association was not statistically significant. All cases recovered without specific therapy. No definite proof could be found to demonstrate the source of the outbreak or the route of transmission but some observations suggesting person-to-person transmission are discussed.