Compositions and methods effective in inhibiting abnormal or undesirable cell proliferation, particularly endothelial cell proliferation and angiogenesis related to neovascularization and tumor growth are provided. The compositions comprise a naturally occurring or synthetic protein, peptide, or protein fragment containing all or an active portion of the C-terminal portion of proteinase inhibitors such as TFPI. The methods involve administering to a human or animal the composition described herein in a dosage sufficient to inhibit cell proliferation, particularly endothelial cell proliferation. The methods are useful for treating diseases and processes mediated by undesired and uncontrolled cell proliferation, such as cancer, particularly by inhibiting angiogenesis. Administration of the composition to a human or animal having prevascularized, metastasized tumors is useful for preventing the growth or expansion of such tumors.