A community-based, double-blind, randomized trial of praziquantel was carried out in an area of Zambia endemic for schistosomiasis. The aim of the study was to assess the impact of the treatment on Schistosoma mansoni morbidity. A total of 377 infected children, aged seven to 19 years, was randomized into two groups: one of 190 (group A) and one of 187 (group B). All children were treated with 40 mg praziquantel/kg at the start of the study. Six months later, the children in group A were re-treated with the same dose of praziquantel, while the children in group B were given placebos. All children were followed up three, six and 12 months after the initial treatment, morbidity being clinically evaluated at the six- and 12-month follow-ups. The results show that, in both groups of children, there were significant reductions in splenomegaly, hepatomegaly, and subjective symptoms of morbidity six and 12 months after initial treatment. However, there were no significant differences, between the two groups, in the prevalences of these symptoms of morbidity. It therefore appears that once-yearly treatment of children, in this and similar endemic areas, is sufficient to reduce schistosomiasis morbidity to, and maintain it at, a tolerable level.