The seroepidemiology of Cryptosporidium infection was investigated in a representative sample of a normal population in the State of Sao Paulo, Brazil using a recombinant form of the immunodominant 27-kDa sporozoite antigen. IgG seropositivity was low in infants following loss of maternal antibody but quickly increased to ~60% by 5 years, then 80% by the age of 10 years, after which prevalence remained constant. The broad range of antibody concentrations is consistent with previous reports that the IgG response to C. parvum is short-lived. There is also evidence that average antibody concentrations increase with age. Results suggest that the recombinant antigen may be a more sensitive method of measuring seroprevalence than the native antigen in Western blot. Although cross-sectional studies can provide an insight into the epidemiology of C. parvum in normal populations, further studies investigating the dynamics of the humoral immune responses to Cryptosporidium and the use of serology in epidemiological studies are required.