During the period from July to November 1984, 265 consecutive febrile infants younger than one year of age were evaluated in a pediatric emergency department. None had a source of infection on physical examination, and all were admitted with the diagnosis of "rule out sepsis." During the month of July, all patients with positive urine culture results had their urine sample collected by bag. In no instance was there a clinical diagnosis of urinary tract infection because of the presence of contaminant bacteria. A program was instituted on August 1, 1984 which encouraged the utilization of either bladder catheterization or suprapubic aspiration techniques, and discouraged bag collection technique for culturing urine. Over the next four months, catheterization and suprapubic aspiration techniques increased from 0 to 72%; bag technique decreased from 100 to 27%. Also, during this period the incidence of urinary tract infection increased to 5.53%. Seventy-five percent of patients with urinary tract infection had an initial urinalysis with less than 5 to 10 white blood cells/HPF, and 60% had an initial urine specific gravity of less than or equal to 1.005. Eighty-seven percent of infants with a positive urine culture result collected by bag technique revealed a mixture of more than three organisms. The technique utilized for collecting urine for culture in infants has a major impact on the incidence of urinary tract infection. The absence of pyuria is not a reliable indicator of the absence of urinary tract infection. Infants with urinary tract infection may have a transient loss in urine concentrating ability early in the course of their infection.