Fifty premenopausal women with uncomplicated urinary tract infection (UTI) were treated with a single dose of 100 mg of ofloxacin. Duration of symptoms before the treatment, number of episodes of UTI during the last year, association with sexual activity, and the use of contraceptives were evaluated in relation to the clinical and microbiological response. Forty-seven women (94%) were cured clinically and microbiologically three days after treatment. Follow up at 28 days disclosed that 43 (86%) were free of symptoms and in 40 (80%) the urinary cultures were negative. Significantly higher rates of microbiological failure or reinfection were found among women with three or more episodes of UTI during the last year. This study shows that a single dose of ofloxacin is effective for therapy of uncomplicated UTI in young women.