The efficacy of lomefloxacin given at 400 mg once daily for 14 days compared with that of trimethoprim-sulfamethoxazole at 160 and 800 mg, respectively, given twice daily for 14 days in the treatment of symptomatic complicated urinary tract infections was studied in a prospective, randomized, single-blind multicenter study. A total of 133 subjects presenting with signs and symptoms of urinary tract infection and an underlying abnormality consistent with complicated urinary tract infection were enrolled in the study. Bacteriologic cure was significantly better in 68 subjects randomized to lomefloxacin than in 65 subjects randomized to trimethoprim-sulfamethoxazole at short-term follow-up (88 versus 52%; 95% confidence intervals [CIs] 77 and 94% and 39 and 65%, respectively) this difference was no longer significant at long-term follow-up (64 versus 47%; CIs, 52 and 75% and 32 and 57%, respectively). Clinical outcomes were similar for both therapeutic regimens at short- and long-term follow-ups. The organisms that infected the subjects pretherapy mere more frequently resistant to trimethoprim-sulfamethoxazole, and drug therapy was discontinued more frequently in subjects treated with trimethoprim-sulfamethoxazole because of adverse antimicrobial effects. In secondary analyses, outcomes did not differ with age or underlying genitourinary abnormality. Lomefloxacin was, however, superior to trimethoprim-sulfamethoxazole for the eradication of gram-negative organisms at both short- and long-term follow-ups (bacteriologic cure rates, 96% [CI, 91 and 100%] versus 70% [CI, 56 and 84%] and 86% [CI, 81 and 91%] versus 59% [CI, 45 and 73%]) and for bacteriologic cure for the 41 men enrolled in the study (bacteriological cure rates, 95% [CI, 73 and 98%] versus 43% [CI, 23 and 66%] at short-term follow-up and 80% [CI, 56 and 93%] versus 25% [CI, 12 and 52%] at long-term follow-up). Lomefloxacin is superior to trimethoprim-sulfamethoxazole for the treatment of symptomatic complicated urinary tract infection and for bacteriologic eradication at short-term follow-up. At long-term follow-up, a bacteriologic benefit remained for eradication of the initial infecting gram-negative organisms and for men.