Neonatal hemangioma is a common benign proliferation of unorganized structures containing stromal and capillary endothelial cells. We tested the hypothesis that such cell proliferation might result from the release by stromal cells of endothelial cell mitogens. Stromal cells cultured from biopsies of surgically removed life-threatening hemangiomas released an endothelial cell mitogen in vitro that was indistinguishable from vascular endothelial growth factor (VEGF) based on independent criteria such as affinity chromatography for heparin or anti-VEGF IgG and radioreceptor assay. A functional product of the KDR gene encoding a cognate VEGF receptor was also expressed by these stromal cells. Transient transfection with antisense oligonucleotides targeted on the translation initiation codon of KDR abolished its tyrosine phosphorylation and mitogenic response of neonatal hemangioma cells to VEGF, confirming the existence of an autocrine loop of proliferation. When grafted in nude mice, these stromal cells elicited an angiogenic response that was blocked by neutralizing anti-VEGF IgG. These results might provide a clue to the importance of stromal cells in the pathogeny of neonatal hemangiomas.