Oral treatment of simple urinary tract infections generally involves 5 to 7 days of antibiotic therapy. This study with enoxacin, a new antibacterial agent of the quinolone-azaquinolone class, investigated the efficacy of a single dose compared with 3 days of treatment. A total of 154 outpatients with symptoms of simple cystitis were treated in an open randomized study with enoxacin, either one 600-mg dose or 200 mg twice a day for 3 days. A urine sample was collected for culture before treatment, 7 to 10 days after treatment, and 4 to 6 weeks after treatment. Seventy-three patients had positive bacterial cultures from the pretreatment urine sample; the predominant pathogen was Escherichia coli, along with a number of other gram-negative organisms and Staphylococcus spp. Of these patients, 33 received a single dose of enoxacin and 40 were treated for 3 days. Follow-up examination at 7 to 10 days showed negative urine cultures in 76% of patients from the single-dose group and 89% from the multiple-dose group, a difference that was not statistically significant (P = 0.665, Fisher's exact test). A number of patients were lost to follow-up at 4 to 6 weeks. However, of those who did attend, only three patients were shown to have relapsed or become reinfected (two in the multiple-dose group and one in the single-dose group). Enoxacin was well tolerated in both groups of patients; the few adverse events were mostly mild.