Egg output, proteinuria, erythrocyturia, and leukocyturia were quantitatively assessed in six schoolchildren with urinary schistosomiasis due to Schistosoma haematobium on 5 days before and 1-35 and 61-65 days after administration of praziquantel (40 mg/kg). Twenty days after therapy, egg output had decreased from a median of 310 before therapy to less than one egg per 10 ml of urine. This decrease was paralleled by a reduction in proteinuria (1.01 g/liter), erythrocyturia (2,142/microliter), and leukocyturia (803/microliter) to normal values. Between days 31-35 and 61-65, five of the six patients had no detectable viable eggs on five consecutive days. Thickening of the bladder wall and vesical polyps detected by ultrasonography returned to a normal appearance one month after treatment. However, dilatation of the renal collecting system showed no significant reduction.