Schistosoma mansoni and S. haematobium infections were studied quantitatively in 400 consecutive autopsies in Cairo, Egypt. Pathological findings were correlated with the presence or absence of schistosome infection as well as with the intensity of infection. The intensity of S. haematobium infection was much greater in cases with obstructive uropathy (hydroureter or hydronephrosis) than in those without; however, infection intensity was similar in cases with mild or severe obstructive uropathy; i.e., the presence, but not the degree, of obstructive uropathy was related to infection intensity. The presence of focal lesions was more important in the pathogenesis of severe obstructive uropathy than was the concentration of eggs in the ureters or bladder. The most severe cases of schistosomal obstructive uropathy were usually caused by ureteral stenosis, ureteral stones or distortion of the ureteral orifices. Pyelonephritis was of similar frequency in cases with and without schistosomiasis. Pyelonephritis occurred most often in patients with bladder outlet obstruction or distortion of the ureteral orifices. In the absence of these lesions, hydroureter did not predispose to pyelonephritis. Glomerulonephritis was not positively associated with S. haematobium infection. S. mansoni infection caused severe schistosomal colonic polyposis in three heavily infected individuals. Both S. mansoni and S. haematobium also caused less marked polyposis. Salmonella infections were infrequent and not significantly associated with schistosomiasis. In this series, schistosomiasis was rare in patients 9 yr or less of age. Among cases over 9 yr of age, schistosomiasis was considered the cause of death in 9.2% of infected cases (6.2% of all cases). Among infected cases, 2.5% of deaths were ascribed to schistosomal obstructive uropathy, 2.4% to bladder cancer associated with schistosomiasis, 3.1% to Symmers' fibrosis of the liver, 0.8% to schistosomal colonic polyposis and 0.4% to salmonellosis associated with schistosomiasis. Although we made no deliberate selection of cases for study, any hospital-based series gathers a highly selected group of patients. Our results should be applied to other infected populations with great caution. In addition, pathologic studies concentrate on serious anatomic lesions and cannot accurately evaluate the effects of other important lesions, such as schistosomal cystitis, which produce considerable morbidity.