Although Schistosomiasis mansoni is generally considered as a disease of severe prognosis, the AA attributed this notion to the fact that doctors living in big cities, farway from the endemic regions, are used to see not more than very advanced and serious cases, the only ones which come to their hospitais. Enphasizing that visíts to or researches on the areas of endemicity give to the doctors a more exact idea of the morbidily of the parasitosis, they present the result of a parasitological and clinicai study performed in a small village (Tiuma) of São Lourenço da Mata, State of Pernambuco, Brazil; 90% of patients with intestinal form (many assymptomatic cases), 7,7% with the hepato-intestinal form, and 2,3% with the hepato-spleno-intestinal form of the disease. No one of the 220 patients had digestive hemorrhage, ascites or hepatic coma during the period of observation. A review of the possible causes of variations in the evolution of the infection ly Schistosoma mansoni was made. The AA admit that the heaviness of the infection (worm burden) and the immunological conditions of each patient are the most important ones.