Bacteriuria was studied in an unselected population of 3,581 infants. Screening was performed at three time intervals during the infants' first year of life. The public Child Health Centers cooperated in the screening and bag samples were obtained from the infants with the parents help. Bacteriuria was verified by suprapubic aspiration. 94% of the infants took part in the screening and bacteriuria was confirmed in 14 girls (0.9%) and 36 boys (2.5%). An additional 20 girls (1.1%) and 20 boys (1.2%) in the study population presented with symptomatic urinary tract infection before 12 months of age. Bacteriuria in boys was predominantly found early in infancy both with screening techniques and through symptomatic urinary tract infections.