METHODS A previously characterized rabbit model was used to study vascularization of the fusion mass in a posterolateral intertransverse process fusion. OBJECTIVE To determine the interosseus origin of the new blood vessels in a posterolateral intertransverse process fusion mass and to test the hypothesis that bone incorporation and the extent of vascularization are closely related. BACKGROUND It has been reported that vascularization is essential for bone graft incorporation. There are, however, few reports dealing with vascularization of the spinal arthrodesis. METHODS Thirty-one adult New Zealand White rabbits underwent bilateral intertransverse process fusion, using autogenous iliac crest bone graft. The rabbits were killed at 3 weeks (n = 6) and 6 weeks (n = 25) after surgery, and colored silicone was injected to fix the vasculature. A semiautomated image analysis system was used to assess the percentage of the area of vascularization in the fusion mass and the transverse processes. RESULTS The major interosseus blood supply for vascularization of the autogenous bone graft came from upper and lower transverse processes. There were three types of fusion mass observed at 6 weeks after surgery: solid type, solid type with cartilaginous cleft, and nonunion type. There was significantly less vascularization of the fusion mass and of the transverse processes in the nonunion type compared with that in the solid type and with that in the cartilaginous cleft type. CONCLUSIONS There is a close correlation between bone incorporation and the extent of vascularization in a posterolateral intertransverse process fusion.