Thirty-seven adult patients with acute urinary tract infections (UTI) were randomized to receive either a seven day (lower UTI) or a 14 day (upper UTI) course of norfloxacin 400 mg orally twice daily, or nalidixic acid 1 g orally four times per day. Mean age, underlying disease and infecting organisms were similar in the two groups. Nine patients in the norfloxacin group and seven in the nalidixic acid group had presumptive evidence of upper UTI. Overall, 12 patients had antibody-coated bacteria-positive infections. The infecting organisms were: Escherichia coli (27), coagulase-negative staphylococci (four), Citrobacter freundii (three), Klebsiella pneumoniae (three), and Proteus mirabilis, Proteus vulgaris, Pseudomonas aeruginosa, Enterobacter agglomerans, Streptococcus agalactiae, Enterococcus faecalis (one of each). All of the organisms were susceptible to norfloxacin, while 81% were susceptible to nalidixic acid. The effects on the periurethral and anal canal flora were similar in both groups. Five patients in each group experienced adverse clinical effects. The cure rates for norfloxacin and nalidixic acid were 79 and 83%, respectively. There were two failures, two relapses and four reinfections in the norfloxacin group. In the nalidixic acid group, there were two failures, one relapse and four reinfections. One of the failure patients in the nalidixic acid group developed resistance to the drug, and two of the four reinfections were due to organisms resistant to nalidixic acid. In this patient population it was concluded that nalidixic acid may be as effective as norfloxacin in the treatment of acute, symptomatic UTI.