A clinical study of 34 previously healthy young patients simultaneously infected in an endemic area of schistosomiasis mansoni is presented, emphasizing the initial phase of the infection. Its intensity was established according to the occurrence, intensity, and duration of the signs and symptoms in order to investigate the possible correlations between the worm burden (estimated by the number of eggs in faeces), the blood eosinophilia and specific levels of IgE (estimated by the area of immediate intradermal reaction), with the clinical manifestations. A significant but low-level association was found between the worm burden and morbidity, suggesting that multiple factors, besides worm burden itself, may contribute to the pathogenesis of the disease.