The study of fracture healing inevitably includes the problem of delayed union since this is the antithesis of the clinician's aspirations in treating the fracture. Recent attention has again been focused on radiation-induced delayed union as one of the rare but intriguing causes (1). Normal long-bone fracture healing requires the development of adequate blood supply, proliferation of normal functioning osteoblast, and the formation of bony callus (2). The stage of initial hemorrhage progresses to organization of fibrin with vascular granulation tissue and fibrocartilage development. This is accompanied by periosteum- and endosteum-osteoblast proliferation. Spongy, new bone formation replaces the fibrocartilage and produces extensive bony callus (3–5). Delayed union may result from impaired blood supply or impaired proliferation of normal-functioning osteoblasts (6–9). Studies of radiation effect on bone growth provide clues to the understanding of radiation-induced delayed union. In growing bone radiatio...