A cross-sectional study of morbidity associated with Schistosoma mansoni infection in an area in North-East Brazil where the disease is endemic was carried out in 1974. The survey was repeated in 1977, before mass treatment with oxamniquine, providing a cohort of 210 individuals who had both examinations. The high prevalence of hepatomegaly (over 80%) and of splenomegaly (over 15%) contrasted with rates of 10% and 1%, respectively, in a non-endemic area. Over the 3-year period hepatomegaly spontaneously regressed in 13% of patients, and splenomegaly regressed in 56%, a phenomenon most common in older individuals with light infections. Those with heavy infections—ie, 500 or more eggs per g faeces, had an excess risk of splenomegaly of 19·6% and, of its persistence, of 61· 5%. Thus, intensity of infection was a critical factor in liver and spleen involvement, and programmes of chemotherapy that reduce infection should mitigate the risk of schistosomal morbidity.