In the case of vascularized bone graft for bone defects of the proximal ulna, there are several options for recipient vessels. However, the radial artery, ulnar artery, and interosseous recurrent artery were reported to have problems. Therefore, we focused on the posterior radial collateral artery at the bifurcation of the profunda branchii artery. We report 3 cases of bone defects of the proximal ulna: two cases of nonunion following high-energy trauma, and 1 of intractable bone cyst treated using the posterior reversed radial collateral artery as the recipient vessel. Bone union was observed in both nonunion cases 6 months after surgery and at 5 months after surgery in the other case. The posterior radial collateral artery was anatomically stable and used as a recipient vessel of the lateral upper arm flap. We conclude that the reversed posterior radial collateral artery is a useful option for the treatment of bone defects of the proximal ulna.