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In Pemba, Tanzania, 2 sequential surveys were made of the prevalence of haematuria in children (aged 5-19 years) in 52 schools, using direct observation of gross haematuria (bloody urine) and chemical reagent strips as indicators of Schistosoma haematobium infection. 24,462 children were examined in the first survey and 25,575 in the second, 6 months later. The prevalence of gross haematuria was initially 15.8% (3876 cases); 6 months later, after a single dose of praziquantel (40 mg/kg), it was only 2.4% (613 cases), a reduction of 84.9%. The total number of cases of haematuria (gross or otherwise) dropped similarly, from 13,920 (54%) to 6638 (26%), a 52.2% reduction. In towns the degree of reduction was positively correlated with the standard of water supply and sanitation available, and the initial prevalence of gross and total haematuria in schools was positively correlated with the prevalence 6 months later. The cost per person reduced from US$ 0.83 in the first survey to US$ 0.48 in the second (excluding field staff wages paid in local currency), the reduction being due to the reduced number of positive persons. This study has demonstrated the feasibility of using indirect techniques on a large scale for the diagnosis of S. haematobium infection to identify schoolchildren requiring treatment, and to monitor its impact on clinical morbidity in a highly endemic area.