Pyelonephritis, which has long had the interest of physicians, has received increased attention during the past decade. The interest is well justified. Pyelonephritis is probably the most common single type of renal disease; it may cause acute and chronic symptoms, have a pathologic effect on kidney functions, interrupt the normal course of physiologic events such as pregnancy, give rise to bacteremia, produce late sequelae such as hypertension, and is the most frequent cause of uremia without previously recognized kidney disease.1-11 Many problems regarding pyelonephritis remain unsolved, but progress has been made in our understanding of the disease process, its recognition, and the principles of treatment. New methods of study and the therapeutic use of new drugs have contributed new data. Continued gain in our knowledge will be assisted if progressively better differentiation can be made among the different kinds of diseases and responses included under the title of pyelonephritis.