The NC1 domain of alpha3 chain of type IV collagen, namely tumstatin, has been shown to display specific anti-angiogenic properties by inhibiting endothelial cells' proliferation and inducing their apoptosis via an interaction with alphavbeta3 integrin. Until now, the tumstatin anti-angiogenic effect has only been shown by in vitro studies or mouse xenograft experiments. In the present study, we examined the expression of tumstatin in relationship with tumor vascularization in 34 bronchopulmonary human carcinomas. We observed a clear association between tumstatin expression and tumor vascularization. Indeed, a strong expression of tumstatin in the tumor environment correlated with a mildly developed vascular network. In contrast, tumstatin was absent or poorly detected in highly vascularized tumors. Moreover, alphavbeta3 integrin and tumstatin colocalized in capillary endothelial cells, suggesting a potential interaction between these 2 molecules. Thus, our results plead in favor of an in vivo anti-angiogenic effect of tumstatin. This factor, largely expressed in well-differentiated lung carcinomas, could indeed reduce tumor vascularization and thereby limit tumor progression.