Giardiasis, a common infection among children, is caused by a flagellated protozoan called Giardia lamblia. It is well known to be contagious in common living places. This is an epidemiologic study investigating the incidence of giardiasis among the siblings of patients with giardiasis living in the same household. Stool samples of 50 patients with a G. lamblia positive sibling in the same household, and 50 patients with a G. lamblia negative sibling in the same household, were examined for giardiasis by saline-Lugol, formalin-ethyl acetate concentration and trichrome staining methods. Other causes of diarrhea were excluded by microbiologic laboratory tests. Thirty-eight per cent of siblings of G. lamblia positive patients and 8% of siblings of G. lamblia negative children were found to be positive for G. lamblia cysts and/or trophozoites. The differences between the groups were statistically significant (P < 0.001). The results suggested that the examination of G. lamblia among the siblings of patients with giardiasis both in the same age group and living in the same household should be considered.