Vesicoureteric reflux in children was originally considered to be a secondary phenomenon, caused by distal obstruction. We now know, however, that most children with vesicoureteric reflux are not obstructed. More recently, urinary tract infection has been thought to be the cause of most vesicoureteric reflux. To test this hypothesis, the results of 601 voiding cystourethrograms with simultaneous culture of the urine (in children), over a 12 month period were analyzed. Of the 34% (204/601) with vesicoureteric reflux, 88% (179/204) had sterile urine and 12% (25/204) were infected. Of the 66% (397/601) without reflux, 90% (357/397) had sterile and 10% (40/397) infected urine. These data do not support the concept that vesicoureteric reflux is secondary to infection in these children. On the contrary, it suggests that reflux and urinary tract infection are independent variables that often coexist. It is concluded that vesicoureteric reflux is a primary phenomenon and is due to incompetence of the ureterovesical junction.