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At present, anthelmintic therapy with praziquantel at a dose of 40 mg/kg of body weight is the recommended treatment for control of urinary tract morbidity caused by Schistosoma haematobium. Although this standard regimen is effective, drug cost may represent a significant barrier to implementation of large-scale schistosomiasis control programs in developing areas. Previous comparison trials have established that low-dose (20-30 mg/kg) praziquantel regimens can effectively suppress the intensity of S. haematobium infection in endemic settings. However, the efficacy of these low-dose regimens in controlling infection-related morbidity has not been determined in a randomized field trial. The present random allocation study examined the relative efficacy of a 20 mg/kg dose versus a 40 mg/kg dose of praziquantel in control of hematuria and bladder and renal abnormalities associated with S. haematobium infection in an endemic area of Coast Province, Kenya. After a nine-month observation period, the results indicated an advantage to the standard 40 mg/kg praziquantel dose in terms of reduction of infection prevalence and hematuria after therapy (P < 0.01 and P < 0.005, respectively). However, the two treatment groups were equally effective in reducing structural urinary tract morbidity detected on ultrasound examination. We conclude that in certain settings, a 20 mg/kg dose of praziquantel may be sufficient in providing control of morbidity due to urinary schistosomiasis in population-based treatment programs.