Forty-three patients suffering from acute or chronic urinary tract infections have been treated with pefloxacin at the Department of Urology of St. Istán Hospital. Twenty-three patients received oral doses (2 x 400 mg), 1 tabl. in the mornings and 1 in the evenings. Eighteen patients were given 2 x 1 intravenous ampoule (2 x 400 mg) dissolved in 250 ml 5% glucose in slow drip infusion. Both the oral and intravenous doses were given in the patients for 10 days. In two patients complications was observed--in one case rhagades were formed on the lip and face due to photo-sensibilization, in the other case the drug administration had to be discontinued because of nausea and vomiting. These symptoms ceased after the discontinuance of the drug. The clinical conditions, the routes of administration, clinical and bacteriological results are summarized in tables. On the basis of a low number of cases it has been concluded that pefloxacin is a new valuable antibacterial drug the use of which resulted in recovery in 15 out of 18 patients' acute and chronic infections of the urogenital system.