Late benefits of remote antischistosomal therapy were estimated among long-term residents of an area with high transmission of Schistosoma haematobium (Msambweni, Kenya) by comparing infection and disease prevalence in two local adult cohorts. We compared 132 formerly treated adults (given treatment in childhood or adolescence > or = 10 years previously) compared with 132 age- and sex-matched adults from the same villages who had not received prior treatment. The prevalence of current infection, hematuria, and ultrasound bladder abnormalities were significantly lower among the previously treated group, who were found to be free of severe bladder disease. Nevertheless, heavy infection was equally prevalent (2-3%) in both study groups, and present rates of hydronephrosis were not significantly different. Therapy given in childhood or adolescence appears to improve risk for some but not all manifestations of S. haematobium infection in later adult life. Future prospective studies of continued treatment into adulthood will better define means to obtain optimal, community-based control of S. haematobium-related disease in high-risk locations.