A transparent acrylic hamster cheekpouch chamber was used to investigate the elaboration of a tumor angiogenic factor (TAF) by human cutaneous neoplasms; direct tumor implantations, transfilter diffusion, and soluble tumor extracts were used in the study. A diffusible and filterable TAF was extracted from cutaneous tumors and produced distinctive patterns of sequential vasodilatation, tortuosity, and neovascular proliferation in the cheek-pouch membrane. Malignant human neoplasms (eg, melanoma, basal cell epithelioma, squamous cell carcinoma, lymphoma) produced striking neovascularization; vascular tumors (eg, Kaposi sarcoma, pyogenic granuloma, vascular histiocytoma) stimulated dramatic hyperemia and ectasia. Angiogenesis was conspicuously absent after implantation of control materials and nevoid or normal cutaneous components (with the exception of epidermis). Tumor angiogenic factor appears to induce direct stimulation of endothelial cell mitosis and may be essential for survival of nutritionally ravenous neoplastic tissues. The interference with TAF has therapeutic implications.