Despite the advent of potent antibacterial agents the frequency of chronic pyelonephritis has not been significantly reduced, and it has remained notably resistant to permanent cure.1,2In a prior study, prolonged treatment with a single agent, although sometimes suppressive, was curative as a rule only if the infecting bacteria were eradicated within the first several days after the initiation of treatment.3Similar results have been observed by others.4Eradication of the primary infecting strain was sometimes followed by the emergence in the urine of a resistant strain or an entirely new species. These circumstances—failure to eliminate the causative organism even after prolonged treatment, mixed infections or emergence of resistant strains, and superinfection with new species during and after treatment—have been suggested as conditions in which a combination of antimicrobial drugs might be indicated.5 This investigation was undertaken to determine whether a combination of several antimicrobial agents designed to inhibit the