Clinical efficacy of sodium imipenem/cilastatin was studied on 45 patients with complicated urinary tract infections, on whom prior antimicrobial agents were ineffective. A 0.5g dose of IPM/CS was administered by drop infusion twice a day for 5 days. The clinical efficacy was evaluated according to the criteria of the Japanese UTI committee. Of a total of 65 strains, 29 strains of gram positive bacteria and 36 strains of gram negative bacteria, were detected as causative microbials. Bacteriologically, 23 strains of gram positive bacteria (79%) and 34 strains of gram negative bacteria (94%) were eradicated following the treatment. S. aureus, E. faecalis and Flavobacterium sp. were less sensitive to IPM/CS. Overall clinical effectiveness rate of IPM/CS in the present study was 87%. Adverse drug side effects were observed in six patients, namely they were eruption, headache and slight elevation of serum transaminase. These findings suggest that IPM/CS is an effective agent for the treatment of complicated UTI even when the prior medicine was ineffective.