The purpose of this study was to clarify the clinical relevance of carbapenem and third-generation cephalosporin treatment for febrile complicated pyelonephritis, which often leads to urosepsis. Parenteral antimicrobial treatment with a carbapenem or third-generation cephalosporin was administered to febrile patients and the treatment was switched to oral antimicrobial agents after they became afebrile. In principle, the duration of the course of antimicrobial chemotherapy was limited to a total of 14 days. Clinically, the success rates were 97.3% in the carbapenem group and 96.0% in the third-generation cephalosporin group. For microbiological efficacy, the success rates were 89.2% in the carbapenem group and 92.0% in the third-generation cephalosporin group. There were no serious adverse events in the course of the study. The treatment regimen with a carbapenem or a third-generation cephalosporin was highly effective for patients with febrile complicated pyelonephritis and was well tolerated. Either of these regimens could become one of the standard treatments for patients with febrile complicated pyelonephritis.