Three South African patients with severe Rickettsia conorii infection had complicated courses of illness with 2 fatal cases and 1 with gangrene of multiple digits. Immunofluorescent organisms of R. conorii were demonstrated in vascular endothelium of brain, leptomeninges, renal glomerular arterioles and capillaries, renal arteries and veins, myocardial capillaries and arteries, pulmonary alveolar capillaries, pancreatic septa, splenic arterioles, and dermis. Rickettsiae were also observed in hepatic sinusoidal lining cells, splenic and lymph node macrophages, and the blood vessels of the partially viable zone of the amputated digits. Pathologic lesions included cerebral and cerebellar perivascular mononuclear leukocytes, mild mononuclear leptomeningitis, glomerular arteriolitis, vascular and perivascular mononuclear cell-rich inflammatory foci in the kidney, pancreas, skin, and myocardium, hepatocellular necrosis, and pulmonary edema. The sites of lesions and rickettsiae showed strong topographical correlation. Thrombi and hemorrhage occurred in a minority of the sites of vascular injury. Rickettsiae were the apparent direct cause of meningoencephalitis, peripheral gangrene, and other foci of vascular injury. Fatal R. conorii infection with disseminated organ involvement emphasizes the pathogenic potential of this disease.