Norfloxacin (MK-0366), a new oral quinoline antimicrobial agent, was shown to have much wider spectrum than nalidixic acid which includes Pseudomonas aeruginosa and gram-positive cocci. Many studies have been undertaken to determine the clinical effectiveness of norfloxacin in patients with urinary tract infections. Seventy-two patients with urinary tract infections caused by sensitivity strains were treated with either 400 mg norfloxacin per 12 hours or nalidixic acid 500 mg every 8 hours for 7 days. Follow-up examinations showed that norfloxacin is more effective than nalidixic acid, 93% and 83%, respectively, for the eradication of bacteriuria and symptoms. Adverse reactions due to norfloxacin were fewer and minor, and the tolerability seemed to be better than that of nalidixic acid.