Repeated selective population chemotherapy of school age children reduces infection and morbidity associated with Schistosoma haematobium infection. To examine the long-term effect of this treatment on susceptibility to re-infection and late disease, a cohort of Kenyans (n = 194) were re-examined for infection and urinary tract morbidity 7-13 years after they underwent annual ultrasonography and treatment for an average of 5 years beginning in 1984 as children. Controls were previously untreated age-matched individuals residing in the same or adjacent villages. The overall prevalence and intensity of infection were equivalent between the 2 groups. In contrast, the prevalence of bladder wall pathology was 11-fold lower in previously treated (1.5%) versus untreated subjects (17%). Severe hydronephrosis was completely reversed. These data demonstrate that treatment significantly reduced urinary tract morbidity despite re-infection, and suggest that the important risk factors for urinary tract morbidity in adulthood are cumulative intensity and duration of infection during early adolescence.