Twenty children with hymenolepiasis, living at a close community and having more than 1000 eggs/g of feces, were selected for the investigation of the therapeutical efficacy and the morphological alterations induced on the H. nana eggs, following the oral administration of praziquantel in two posologies. The patients were allocated into two groups having equal number of cases. Group "A" was treated with a single dose of 25 mg/kg BWT and group "B" with the same dose but repeated 10 days afterwards. From the third day on until the 19th day it was found, in both groups, the uprising, with a peak around the seventh day, of distorted eggs. The normal eggs decreased gradually. They disappeared in 100% of the cases in group "B", from the 19th day on, but remained present in 20% of the cases in group "A". The tolerance to the drug was excellent with either dose schedule. It was concluded that praziquantel is highly efficaceous in the treatment of hymenolepiasis and that this drug should be administered in two doses of 25 mg/kg, 10 days apart, for those patients presenting intense worm burden and living in close communities.