The prevalence, incidence and reinfection of giardiasis were studied in 365 children attending three day care centres (DCCs) in Havana City. Three stool samples were obtained from each child every 6 months during an 18-month period. We identified three distinct groups of children according to their patterns of infection. In the largest group (51%) children were never found infected. In the second group, they were found infected once or twice during the study period, and in the third and the smallest group (9%) they were found infected in most or all the study periods. This last group seems to be children 'predisposed' to Giardia lamblia infection. The prevalence of giardiasis (20%) remained almost constant throughout the study period. The incidence declined from 16 to 11%, and reinfection increased from 36 to 49%. All the children had normal nutritional status and the only clinical manifestation that correlated strongly with the frequency of cross-sectional surveys positive to Giardia was the number of diarrhoeal episodes recorded during the last 6 months of the study period. Further studies will be necessary to ascertain the causes that determine the 'predisposition' to giardiasis in children.