Cefroxadine and cephalexin were compared in a double-blind study in patients with complicated urinary tract infections. Patients were treated orally with 1 g t.i.d. for five days. Bacterial counts in the urine were determined 1, 3 and 8 hours after the first dose and on the second, third and fifth day of treatment. A decrease in the bacterial population below 10(4) organisms/ml of urine was taken as being indicative of chemotherapeutic effect. Urine specimens were inactivated enzymatically before culture to eliminate any residual cephalosporin. A significant reduction in the bacterial count was seen one and three hours after the first dose, and, to a much greater extent, after eight hours. Reduction in the bacterial count was more frequent in the group treated with cefroxadine. The results during the first eight hours indicate that cefroxadine kills more rapidly than cephalexin, which is in agreement with the experimental findings. Whether the transient differences observed in the rates of inhibition are of clinical relevance remains uncertain.