An epidemiological study was conducted as an initial phase of a longitudinal study on the immunology of Schistosoma haematobium in Upper Egypt. The study area was confined to three villages in Qena governorate, a region endemic solely for S. haematobium. In a cross-sectional survey in two of these villages, the overall prevalence of infection was 28.7% (29.9% at Khozam and 26.9% at El Ayaisha) based on urine examination of the family members in every third household. S. haematobium infection was found in all age groups including infants below 1 year of age. The age distribution of those infected showed the classical rise in the percent infected, reaching a peak of 57-63% in the 11- to 15-year age group followed by a rapid decline and stable low prevalence below 28% in adult years. Males consistently had higher prevalence rates than females at all ages with the profiles of the age prevalence curves strikingly similar. In the two villages, significantly (P less than 0.05) higher infection rates were found in farmers compared to those in other occupations. The overall prevalence of infection in school children (5-16 years old) in the three villages was 61.1% based on the examination of one urine sample. Almost 10% greater prevalence was found when four consecutive daily urines were examined compared to one in the same study group sampled 1 month apart. There was a rapid increase in prevalence with age, reaching a maximum at age 10 with relatively little fluctuation to age 16 years. When comparing the age prevalence data with the intensity of infection expressed as either mean or median age excretion per milliliter urine, it was found that both were parallel and rose sharply from age 5-10 years, but the latter significantly declined several years prior to any perceptible diminution in prevalence.