Rat sciatic nerves were used in a comparative study of vascularized and free (nonvascularized) nerve grafts transplanted into silicone tubes. A total of 39 sciatic nerves were used, 21 as vascularized nerve grafts and 18 as free nerve grafts. The rats were killed at intervals from the first to the 24th postoperative weeks, and biopsied sciatic nerves were processed for morphometric studies using a fiber caliber analyzer. There was a significant increase in the number of large myelinated axons (more than 5 mu) in the vascularized nerve graft models over those in the free nerve graft models at the distal ankle region. Furthermore, the diameters of myelinated axons in the vascularized nerve graft were larger than those in the free nerve graft at all specimen sites during all postoperative weeks. We suggest that the preservation of the vascular system in vascularized nerve grafts would decrease the likelihood of fibrosis and result in better regeneration of axons.