In a prospective, randomized, double-blind trial, ciprofloxacin (250 mg orally, twice daily) was compared with cinoxacin (500 mg orally, twice daily) in 60 patients with urinary tract infections. Most patients were women with uncomplicated urinary tract infections. Escherichia coli was the most common isolate (36 patients). Clinical and microbiologic cure occurred in 20 of 24 (83 percent) evaluable patients treated with ciprofloxacin, compared with 15 of 21 (71 percent) evaluable patients treated with cinoxacin. Ciprofloxacin was well tolerated and had a low incidence of minor side effects that included Candida vaginitis, headache, and gastrointestinal intolerance. Relapse or failed therapy was not associated with the development of resistance.