Angiogenesis is considered essential to fracture healing, but its role in the healing process remains poorly understood. Angiogenesis inhibitors, which block new blood vessel formation by specifically targeting vascular cells, are currently under development for use in cancer chemotherapy, and are potentially powerful tools for defining the consequences of angiogenic impairment on fracture healing. In this study, we directly tested the effects of the angiogenesis inhibitor TNP-470 on the healing of closed femoral fractures in an established rat model system. Beginning 1 day after fracture, animals received either angiogenesis inhibitor at a therapeutically effective antitumor dose, or a weight-adjusted amount of carrier vehicle. The progress of fracture healing was assessed at weekly intervals for 21 days by radiography and histology; functional assessment was carried out at day 24 by biomechanical testing. By all three criteria, treatment with the angiogenesis inhibitor completely prevented fracture healing. Formation of both callus and periosteal woven bone were suppressed, indicating that both the intramembranous and endochondral pathways of osteogenesis were affected. The resulting tissue resembled "atrophic nonunions" often seen clinically in cases of failed fracture healing, but rarely achieved in animal models. These results show that angiogenesis is essential to very early stages of fracture healing, and suggest this model system may be useful for understanding the mechanisms underlying fracture nonunions due to vascular impairment. Finally, the data raise the possibility that impairment of fracture healing may be an adverse effect of clinical treatments with antiangiogenic drugs.