Sixty-three girls with covert bacteriuria were included in a controlled trial of therapy. Recurrent infection in the treated group was common and was not significantly different from the rate of persistent infection in the untreated control group. Two children in each group developed clinical pyelonephritis; the others have remained healthy and all of them have a normal rate of growth. 2 years after diagnosis three of the thirty-four children in the control group and one of twenty-six children in the treated group have radiological evidence of new scars of pyelonephritis. These changes were relatively minor and in both groups of children renal growth was similar to that in normal children. It is suggested that for most of these children therapy is not essential, and that when renal changes occur they are of little or no significance. Prescriptive screening for cobert bacteriuria of childhood cannot be recommended at present.