Controlled laboratory data demonstrate biologic and mechanical characteristics of orthotopically placed canine ulnar autografts. The pattern of bone repair was similar in vascular and nonvascular ulnar grafts beginning with resorption followed by appositional reactive bone formation. The time intervals in the sequence were accelerated in the vascularized grafts. Both the strength and stiffness of the vascularized grafts were found to be significantly greater in Groups studied from six weeks to six months postoperation. There was no statistical difference between the grafts for mechanical testing performed on dogs either one week or one year postoperation. The vascularized ulnar bone grafts fared better than comparably sized nonvascularized grafts and were more rapidly repaired. The mechanical testing demonstrated superior strength and stiffness of the vascularized grafts throughout the repair process.