Eosinophiluria, as quantified by measuring eosinophil cationic protein (ECP) in urinary extracts, microhematuria, egg excretion, and ultrasound-detectable bladder pathology were recorded in Schistosoma haematobium-infected Tanzanian school children at a baseline survey and during an 18-month post-treatment follow-up study. Significant correlations were seen between urinary ECP levels, intensity of infection, and bladder pathology. Treatment resulted in a marked reduction in prevalence and intensity of infection, in a delayed and less marked reduction in ECP levels, and in a resolution of pathology. The overall diagnostic efficiency of the ECP test (cut-off value for the ECP > or =5 ng/ml) in relation to infection was comparable with that of egg count and microhematuria, but with a better sensitivity than a single egg count. In relation to bladder pathology, the diagnostic performance of the ECP test (cut-off value for the ECP > or =25 ng/ml) exceeded that of a single egg count. In addition, the ECP was better in discriminating between different grades of bladder pathology. The present study points to the ECP as a useful marker of both S. haematobium infection and of associated bladder morbidity reflecting the inflammatory status of the bladder wall.