Clinical gastro-intestinal manifestations were studied in 34 patients in the initial phase of schistosomiasis mansoni. The patients, all men, were of similar age and in similar nutritional condition and had been infected simultaneously at the same transmission site. Most (85%) showed some gastro-intestinal sign or symptom, generally of light or moderate intensity; 56% had liquid or pasty diarrhoea, 41% abdominal pain, 29% hepatomegaly, 21% dysentery, 15% anorexia, 12% pain on colon palpation and 9% nausea and/or vomiting. High worm burden was associated with blood in faeces but apparently not with any other clinical manifestation. There was no apparent association between any clinical manifestation and peripheral-blood eosinophil counts or titres of IgE specific for Schistosoma mansoni (evaluated by the area of immediate intradermal reaction to injected adult worm antigen). The absence of association between worm burden and nearly all the clinical gastro-intestinal manifestations strengthens the concept that factors other than worm burden, such as host reactivity, constitute important pathogenetic elements in the initial phase of schistosomiasis mansoni.