We conducted a longitudinal study on giardiasis in three daycare centers in Havana City for a period of 18 months and described a group of children with a "predisposition" or tendency towards re-infection with Giardia lamblia. This group was found to be more frequently associated with clinical symptoms such as diarrhea. A case-control study was designed to determine whether socioeconomic factors and hygiene were associated with this phenomenon. We found no differences between the groups with regard to overcrowding rates, number of persons per bed, absence of certain electric appliances, mother's schooling, or mean family income. However, there were proportionally more fathers with less than complete secondary education among cases as compared to controls. In addition, we found no differences in lack of hand-washing before eating and after defecation; however, we found a higher percentage of families who washed vegetables insufficiently among predisposed children. Finally, a lower percentage of families with predisposed children boiled their water. Our results demonstrate the important role of water as a vehicle for transmission of giardiasis and the importance of various epidemiologic factors.