One hundred and fifty-five Hymenolepis nana-infected children were treated with a single oral dose of praziquantel, a new and effective isochinolinpyrazin compound with a broad spectrum of activity against cestodes. The children were distributed in three groups which received different dosages of the drug. Parasitological cures were obtained in 64 (98.5%) of 65 children given 25 mg/kg body weight, in 61 (93.8%) of 65 given 15 mg/kg, and in 19 (76.0%) of 25 given 10 mg/kg. Durg tolerance was good, and no clinical side effects were observed. A series of blood and urine tests performed before and after treatment in 30 patients who received 25 mg/kg remained within normal values.