Loss of the radius with open fractures of the forearm is rarely reported in pediatric cases and to the authors' knowledge, cases of segmental loss of the distal half of the ulna have not been previously reported. A 6-year-old girl was admitted with a Gustilo-Anderson type IIIB open forearm fracture and loss of the distal half of the ulna after a motor vehicle accident. Serial debridement was performed and a Kirschner wire was inserted into the distal half of the ulna. Unexpectedly, the ulna regenerated and the defect healed 4 weeks later. Soft tissue coverage was achieved with a skin graft and uneventful healing ensued. Eight weeks later, the bone was spontaneously and completely reconstituted. This case report demonstrates that substantial defects of the ulna may be spontaneously reconstituted over time. Copyright (C) 2021 by the American Society for Surgery of the Hand. All rights reserved.