Though Cryptosporidium spp. is considered to cause only a self-limiting illness in immunocompetent children, data from Guinea Bissau suggest that cryptosporidiosis may be a significant cause of deaths in developing countries. An impaired cellular immune function could explain the severe course of cryptosporidiosis in these children. We therefore investigated in a community study whether pre-infectious CD4/CD8 status had an impact on incidence and severity of cryptosporidiosis. Of 168 children below two years of age 21 experienced Cryptosporidium-infection within 156 days after blood sampling, but no tendencies of pre-infectious impaired cellular immune function was found in the cases compared with controls, nor did nine children who acquired persistent diarrhoea or three who died have impaired CD4/CD8 status.