Four patterns of tissue involvement can be distinguished in sepsis due to gram-negative enteric bacilli. When intense local inflammation predominates, cellulitis or thrombophlebitis results, often with venous or arterial obstruction. Bacteria are present in the affected tissues, but not in sufficient numbers to be seen microscopically. When bacterial proliferation is unchecked by an appropriate leukocyte response, ecthyma gangrenosum, erythema multiforme, or diffuse bullous lesions may occur with minimal clinical or histologic signs of inflammation. In symmetric peripheral gangrene associated with disseminated intravascular coagulation, bland fibrinous deposits are seen in small vessels but neither inflammatory cells nor bacteria are present. The fourth kind of lesion is that seen in bacterial endocarditis. In all four patterns a vascular component is prominent clinically and histologically. The pathogenesis of these lesions is multifactorial; in each individual case the interaction between bacterial and host factors probably determines which clinical picture will result. The appearance of symmetric soft tissue lesions of the extremities in the absence of predisposing local conditions suggests the possibility of sepsis due to gram-negative bacilli, especially if other clinical features indicate that sepsis might be present.