Hemangiomas are vascular tumors characterized by rapid growth, increased endothelial turnover, and increased numbers of mast cells. Vascular malformations grow commensurately with the patient or expand secondary to hemodynamic alteration and are characterized by a normal endothelial cell cycle and normal numbers of mast cells. Operative specimens of vascular birthmarks were categorized as hemangiomas or malformations based on clinical history, light microscopic examination, and mast cell quantitation. The specimens were cultured in tumor-conditioned medium and plasma clot culture. Capillary endothelium derived from hemangioma specimens formed capillary tubes in tissue culture--"angiogenesis in vitro." Capillary endothelium from malformations was difficult to culture and was not observed to form tubules. Hemangioma specimens demonstrated rapid outgrowth of tubular structures from plasma clot cultures, whereas malformation tissue did not produce such outgrowth. These results indicate that hemangioma endothelium grows preferentially in culture in comparison with endothelium from vascular malformations. It suggests a biologic difference, which correlates with our previously proposed classification.