Thirty-two patients (18 men and 14 women), who ranged in age from 28 to 91 years (mean, 71.2 years), with urinary tract infections caused by Pseudomonas species or other organisms resistant to trimethoprim-sulfamethoxazole were treated with 500 mg of orally administered ciprofloxacin every 12 hours. Thirty patients completed at least five days of therapy and were evaluated for efficacy. Of these, the treatment of 28 (93 percent) patients was considered successful, with urine cultures yielding negative results five to nine days after cessation of therapy. Three of these patients were found to be reinfected with their primary pathogens when culture specimens were obtained again three to four weeks later. The two patients who received treatment that was classified as having failed had urine cultures that persistently grew Pseudomonas aeruginosa. Superinfections occurred in eight patients, four with diabetes and four with underlying central nervous system disease. Adverse reactions required discontinuation of therapy in two patients. Although the rates of reinfection and superinfection were somewhat high, these patients had a high frequency of underlying diseases that predisposed them to recurrent or difficult-to-treat infections. Despite these shortcomings, ciprofloxacin is a welcome addition to the oral antibiotic regimen for the treatment of antibiotic-resistant urinary infections.