The optimal timing of intervention in non-maturing AVFs remains controversial, and can only be definitively addressed by a randomized clinical trial (RCT) comparing early (6 weeks) vs late (3 months) angioplasty of non-maturing AVFs. The investigators will perform a single-center, prospective RCT. The investigators will recruit 112 patients who initiated hemodialysis with a CVC, then had a new AVF created after commencing dialysis, and then had a 6-week postoperative ultrasound that revealed an immature AVF (diameter < 4 mm diameter or blood flow < 500 ml/min). These patients will be randomized to one of two groups: an early intervention group that will undergo a routine PTA at 6 weeks after AVF creation, or a late intervention group in which early PTA will be avoided and subsequently be performed only if the 3-month ultrasound indicates persistent AVF immaturity. These patients will be followed for 2 years.