The prevalence and morbidity of schistosomiasis haematobium were studied in three geographically different villages in Giza governorate in Egypt. The survey included 786 children ranging in age from 6 months to 10 years. There was a marked variation in the prevalence of infection in the three areas, correlated with the socioeconomic standards and sanitary habits, emphasizing their role in the spread of the disease. The youngest child infected was a 14-month-old boy, while the peak of prevalence of infection was in children 8 to 10 years old. The intensity of infection, as indicated by the heaviness of urinary egg output (mean of 3 successive days), was unrelated to the degree of prevalence in the area concerned. On the other hand, such intensity of infection correlated well with the severity of the clinical picture. Immediate skin-test reactivity was related to the intensity of infection, though unreliable as a single diagnostic test in children. Delayed reactivity was less sensitive. Stool examination showed a higher prevalence of other helminthic infections in schistosome-infected children aged 6 to 10 years than in uninfected children. The impact of schistosomiasis on the general health of children as shown by the anthropometric measurements was mild, except in heavily infected children in whom it was significant. The study focused on the overshadowed young age period when infection can be radically cured and even easily avoided. The data point to the importance of considering the intensity of infection, and not the prevalence, in weighing the need for therapy against its toxic effects in children, or in discussing the priorities of treatment in an endemic area.