In the Assiut, Egypt Epidemiology 1, 2, 3 investigation, a sample of 14,204 persons in 10 villages, 31 ezbas (satellite communities), and 2,286 households was drawn from a rural population of 1,598,607. Parasitologic examination of urine and stool were made for Schistosoma haematobium and S. mansoni, and physical and ultrasound examinations were made on a 20% subsample. The overall estimated prevalence of S. haematobium was 5.2 +/- 0.5 (+/- SE). This varied considerably by village and ezba, ranging from 1.5% to 20.9%, with ezbas having a slightly higher overall prevalence than villages. The overall estimated geometric mean egg count was 6.6 +/- 0.5 eggs per 10 ml of urine and was consistently low throughout the communities. Infection with S. haematobium was associated with age (peak prevalence of 10.6 +/- 1.5% in 15-19-year-old age group) males, children playing in the canals, a history of blood in the urine, and reagent strip positivity for hematuria and proteinuria. The prevalence of either hepatomegaly or splenomegaly detected by physical examination was low (4.0% and 1.5%, respectively). The prevalence of hepatomegaly determined by ultrasonography was substantially higher, 24.1%. The prevalence of periportal fibrosis (PPF) was 12.0%, but grade II or III PPF was present in less than 1%. Ultrasonography-determined hepatomegaly, in both the midclavicular line and the midsternal line, increased by age to more than 30%. Periportal fibrosis was more common in the age groups in which infection rates were the highest. At the village and ezba level of analysis, the prevalence of hepatomegaly, splenomegaly, and PPF tended to be higher in communities having the highest prevalence of infection with S. haematobium.