The effectiveness and safety of ciprofloxacin, a new quinolone antibiotic, were prospectively evaluated in the treatment of patients with complicated urinary tract infections caused by gram-negative organisms resistant to trimethoprim/sulfamethoxazole. Twenty-five elderly (mean age, 70.4 years) patients (24 men and one woman) were enrolled. Initial pathogens included Pseudomonas aeruginosa (15 isolates), Escherichia coli (five isolates), Enterobacter aerogenes (one isolate), Citrobacter freundii (one isolate), Serratia species (two isolates), Proteus vulgaris (one isolate), and enterococcus (one isolate). Patients received 500 mg of ciprofloxacin orally twice daily for one week (mean, 6.98 days). Results of urine cultures obtained during therapy were negative in all cases, and at one week post-therapy, 21 of 25 (84 percent) infections were cured. Four patients experienced a relapse with P. aeruginosa. Repeat urine culture specimens were obtained at four to six weeks from 14 patients who had cures at one week post-therapy, and seven continued to have cures. Three patients had late relapses with P. aeruginosa, E. coli, or Serratia marcescens, and four were reinfected with new strains. Five patients who received concomitant oral antacids had lower mean peak and trough serum ciprofloxacin levels than did patients not receiving antacids (p less than 0.05, Wilcoxon rank sum test). Mild adverse effects were seen in seven patients: eosinophilia (one patient), eosinophilia and reduced white blood cell count (one patient), crystalluria (one patient), granular casts (one patient), elevation of serum creatinine and blood urea nitrogen levels (one patient), and nausea (two patients), but none warranted discontinuation of ciprofloxacin therapy. P. aeruginosa isolates from two patients who experienced a relapse showed increases in minimal inhibitory concentrations from 0.13 to 0.5 and 2.0 micrograms/ml, respectively, to ciprofloxacin and other antibiotics. Orally administered ciprofloxacin was a safe and effective therapy for complicated urinary tract infections in elderly patients.