Ten patients with chronic urinary tract infection by Pseudomonas aeruginosa or Proteus species were treated with carfecillin. Excellent response was seen in one case, fairly good in 5 cases and clinical effective rate was 60%. 2. The recurrence suppression effect of carfecillin was examined in two patients suffering from complicated urinary tract infection. In both cases, recurrence was not found when carfecillin was administered for a week after the effective pretreatment of carbenicillin, but found when administered for two weeks. 3. Side effects were observed in 3 cases (itching, tinnitus, diarrhea).