In 50 of 100 children with clinical giardiasis studied, the presence of Giardia lamblia was proven by stool and/or duodenal aspirate examination; the presence of the parasite was not demonstrated in the rest. Serum antibodies to G. lamblia were investigated in all children by an indirect immunofluorescent test (IIF) with G. lamblia trophozoites as antigen. It was found that all the children with positive duodenal aspirate had positive serology (titers ≥ 1/32). The best correlation between parasitological and serological procedures were in the 1–5-year-old age group, and there was an age dependent increase of the antibody titer. Most of the parasitologically negative children were serologically negative. Our results support the use of IIF as a useful diagnostic procedure in the diagnosis of giardiasis in children.