73 boys who presented to their general practitioners over the course of one year with symptoms of urinary-tract infection and were found to have bacteriuria were referred to a three-year prospective study. This included clinical and radiological investigations and monitoring of the preputial flora and midstream-urine culture at monthyl intervals. 22 (30%) of the boys had radiological abnormalities of the urinary tract; 6 had pyelonephritic changes. Only 2 required urinary-tract surgery. Several findings of the study suggest that the natural history of the disease in boys is different from that in girls. Proteus spp.predominated as the infecting organisms. Culture of swabs from the preputial sac, and comparison with matched controls, suggested that the source of infection in boys is the prepuce or urethra rather than the bowel as in girls. Recurrence of infection, in the absence of radiological abnormality, was rare; 51 boys (70%) had no recurrence throughout the follow-up period. There was evidence that recurrence in boys is related to the persistence of gram-negative organisms in the urethra, revealed by low bacterial counts on midstream culture, and it is suggested that in boys urethral infection may be as important as bladder bacteriuria.