From January to December 1987, faecal specimens received from 3186 white children and adults with diarrhoea were examined for the presence of cryptosporidial oocysts. They were found in 129 patients, an incidence of 4·1 %. Most of the infections were in children less than 5 years of age, giving an incidence of 6·2% in this age group. Cryptosporidium was usually the only pathogen detected, additional pathogens being identified in only 19 cases. A marked seasonal increase in incidence was seen in 107 cases (82·9%) between January and May, that is in late summer and early autumn in South Africa.