In a small community in Maniema, Zaire, heavily infected with Schistosoma mansoni, direct observations were made of the side effects of praziquantel in the hours immediately after treatment. Intense abdominal discomfort and the production of bloody diarrhoea were observed in more than half of the treated population. These effects were seen both in children and in adults and the onset of the symptoms was registered within 30 min of treatment. The frequency of the side effects was correlated with the intensity of the infection. No satisfactory explanation could be given why the side effects in Maniema appear to be so intense compared with the general experience elsewhere.