Morbidity of Schistosoma mansoni infection was assessed in 536 infected Sudanese schoolchildren using an ultrasonographical staging system for periportal fibrosis of the liver. S. mansoni ova excretion in stools was mild in 28%, moderate in 58%, and severe in 14% of the patients. Grade 1 periportal fibrosis was found in 10.3%, grade 2 in 23.1%, and grade 3 in 4.7% of S. mansoni-infected children. Girls and boys were equally affected. The distribution of periportal fibrosis compared to the age of patients was homogeneous. There was no significant difference of the rate of periportal fibrosis in the 3 groups of intensity of infection. However, in the 204 patients who showed periportal fibrosis, there was a strong relation between severity of ultrasonographically detectable fibrosis with intensity of egg excretion. In 332 children, of which 73 had a heavy infection, no signs of periportal fibrosis were detected. Sudanese children show a variable susceptibility towards the development of S. mansoni-induced periportal fibrosis. High risk patients can be detected at a childhood stage by measuring the intensity of infection and investigating the liver by ultrasound.