Few competent clinicians would overlook the classical picture of pyelonephritis as an acute infectious disease. Chills, fever, malaise, and costovertebral-angle tenderness provoke a well-conditioned diagnostic reflex. Paradoxically, however, pyelonephritis continues to be a major "surprise diagnosis" at the autopsy table or under the microscope which is thoughtfully trained on biopsy material or freshly collected urinary specimens.1 Surveys have shown that less than one in five cases of pyelonephritis is diagnosed before death. No figure can be offered for the number of patients with pyelonephritis in whom the diagnosis was made too late for effective therapy. I believe this situation obtains not only because of the prolonged and insidious nature of the disease process but also of its protean clinical manifestations.2 Pyelonephritis is, in fact, one of our great imitators. The present study will present a brief classification of pyelonephritis, a review of known etiologic and bacteriologic factors, a