Twenty-four patients with complicated urinary tract infections (UTI) were received ceftibuten (CETB) 400 mg b.i.d. orally for 5 days, and the following results were obtained. One of the recipients was excluded for negative bacteriuria before treatment. According to the criteria for evaluation of clinical efficacy of antimicrobial agents on UTI, 23 patients were assessed: 9 of them were judged to show excellent responses, 6 to have moderate responses and 8 poor responses. The overall efficacy was 65%. Four patients with indwelling catheter failed to show positive clinical response, but 15 out of 19 patients without indwelling catheter responded positively. As to bacteriological responses, organisms included Gram-positive cocci were eradicated in 28, replaced in 9 and persisted in 6 strains. Of 24 Gram-negative rods except 1 strain of Pseudomonas sp., 23 (96%) were eradicated. Safety evaluation was made in all 24 patients received CETB. No adverse reactions were observed in any cases. As to laboratory test results, slight, transient elevations of GOT, GPT and Al-P of serum were noted in 1 patient. Their values, however, returned to normal levels in 4 weeks.