One hundred and eighty patients from the "Clínica de Doenças Infecciosas e Parasitárias do Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo" with mansonic schistosomiasis have been treated with oxamniquine (single oral dose 12.5 — 15 mg or 16-20 mg/kg body weight, respectively to patients younger or older than 15 years old). The patients were 5 to 65 years old and the predominant clinical forms were intestinal and hepato-intestinal disease. The main neuropsychiatric side effects were: drowsiness (50.6%), dizziness (41.1%), headache (16.1%), temporary amnesia (2.2%), behaviour disturbances (1.7%), chills (1.1%), seizures (1.1%). In 20 patients the neurotoxicity associated with the drug has been evaluated comparing the electroencephalogram before and after the treatment. Alterations have been detected in 3 (15%) but were not associated with neuropsychiatric manifes- tations. The results show that oxamniquine determines toxic side effects in the neuropsychiatric area.