We demonstrate a physiological role for tumstatin, a cleavage fragment of the α3 chain of type IV collagen (Col IVα3), which is present in the circulation. Mice with a genetic deletion of Col IVα3 show accelerated tumor growth associated with enhanced pathological angiogenesis, while angiogenesis associated with development and tissue repair are unaffected. Supplementing Col IVα3-deficient mice with recombinant tumstatin to a normal physiological concentration abolishes the increased rate of tumor growth. The suppressive effects of tumstatin require αVβ3 integrin expressed on pathological, but not on physiological, angiogenic blood vessels. Mice deficient in matrix metalloproteinase-9, which cleaves tumstatin efficiently from Col IVα3, have decreased circulating tumstatin and accelerated growth of tumor. These results indicate that MMP-generated fragments of basement membrane collagen can have endogenous function as integrin-mediated suppressors of pathologic angiogenesis and tumor growth.