An unusual case of hemangiomatosis is presented involving apparently independently both the flexor muscles of the forearm and ulna in a white girl thirteen years old. The patient did not present any of the usual clinical symptoms of hemangiomatosis of muscle or bone. The roentgenographic changes in the ulna were not those usually associated with hemangiomatosis of tubular bone. Pathological sections showed typical hemangiomatosis involving the flexor muscles as well as the ulna although the changes in the ulna were not as extensive as those in the muscle. At operation the lesion of the flexor muscle did not appear to have invaded the ulna since it was possible to remove the entire flexor muscle mass without difficulty and the thickened periosteum appeared to be intact.