A group of 1,617 infants and 1,711 preschool children were studied for symptomatic and asymptomatic urinary infection and followed up for three to five years. Asymptomatic bacteriuria was found in 1.8% of female infants, 0.5% of male infants, 0.8% of preschool girls, and none of the preschool boys. Seventeen percent of the infants and 13% of the preschool children with urinary infection studied roentgenographically had upper tract damage; 46% of the infants and 9% of the preschool children had vesicoureteral reflux. Infants with normal urinary tracts and urinary infection with or without reflux tended to have recurrent infection, whereas the kidneys remained anatomically normal. The infants with high-risk lesions, such as obstructive uropathy and vesicoureteral junction ectopia and deformity, had substantial bacteriuria on screening culture.