A study of morbidity in Schistosoma mansoni infection was made in 593 Sudanese patients seen in a four-year period in Khartoum Civil Hospital. Clinical and laboratory findings were compared in three egg-count groups and in four clinical forms of the infection. Patients were divided into three levels of intensity of infection: light (up to 100 eggs/gram of stool), moderate (101--400 eggs/g) and heavy (more than 400 eggs/g). According to the presence or absence of visceral enlargement, infected subjects were divided into one of four clinical forms: intestinal, hepatic, hepatosplenic and splenic. Among the symptoms only the passage of blood in the stools was significantly related to intensity of infection, and fever was significantly related to the presence of hepatosplenic disease. Hepatomegaly and splenomegaly were significantly more frequent in the heavy infection group. Anaemia, eosinophilia, raised ESR and an increase in both serum alkaline phosphatase and serum globulins were significantly related to the intensity of infection. On the other hand, haematological and biochemical changes, as well as histopathological changes, were more marked and severe in patients with hepatosplenic disease. For comparison, the findings of 117 patients with S. haematobium infections and of 41 with dual S. mansoni/S. haematobium infections are included.