The pop-off mechanisms that sometimes occur with posterior urethral valves have well recognized implications for renal function, such that 1 or both kidneys can be protected from the deleterious effects of elevated bladder pressures. What has not been defined is the significance, if any, of pressure pop-offs to the developing bladder and ultimate bladder function. To answer this question we reviewed the records of 63 boys with posterior urethral valves. Eight boys who were not yet toilet trained had unevaluable bladder outcomes. Of the 55 remaining boys there was 1 or more pressure pop-off in 39 (71%), including massive (grade 5) reflux, massive reflux associated with ipsilateral renal dysplasia, a patent urachus, large diverticula and urinomas. Bladder outcomes were judged as favorable or unfavorable on the basis of urodynamic parameters and/or patient clinical status. A statistically significant positive correlation was found between the presence of a pop-off and a favorable outcome. Favorable characteristics were found in 34 of 39 bladders (87%) with pressure pop-offs. In contrast, only 9 of 16 bladders (55%) without pressure vents had favorable outcomes. In addition, 5 of the remaining 7 boys without pressure vents have required augmentation cystoplasty. A direct correlation between absolute number of pop-offs and favorable bladder outcome was also noted, further emphasizing this relationship. The presence of a pressure pop-off is a favorable prognostic sign for ultimate bladder function in boys with posterior urethral valves.