Review of clinical and pathologic data from ten fatal cases of Rocky Mountain spotted fever (RMSF) revealed the importance of acute renal failure in the clinical course and of multifocal perivascular interstitial nephritis as the principal pathologic lesion. In nine cases, Rickettsia rickettsii were demonstrated by immunofluorescence in the areas of vasculitis. Evidence was lacking for the role of disseminated intravascular coagulation, glomerulonephritis, or myoglobinuria in the pathogenesis of acute renal failure in these cases. Rickettsia-induced vascular injury led to acute renal failure by several mechanisms. Hypovolemia early in the course resulted in reversible, prerenal azotemia. Transient hypotension in midcourse produced acute tubular necrosis. In fulminant cases, preterminal circulatory collapse was associated with coma and oliguria. The interstitial nephritis could not be demonstrated conclusively to contribute to the acute renal failure.