In an unselected population of 3,581 infants, 14 girls and 36 boys were found to have bacteriuria verified by suprapubic aspiration. Among the bacteriuric infants, one girl and one boy developed symptoms of pyelonephritis close to the time of detection; the others remained asymptomatic. Eleven girls and 34 boys were left untreated. One of these girls and 7 of the boys became abacteriuric after treatment for respiratory tract infections. Ten girls and 26 boys became spontaneously abacteriuric, with a median persistence of bacteriuria of 2 months in girls and 1.5 months in boys. Recurrences were observed among boys only in those who had been treated with antibiotics.