Background: The International Society for the Study of Vascular Anomalies classification is an updated biologic classification that distinguishes vascular tumors from vascular malformations on the basis of clinical and histologic characteristics and biologic behavior. Objective: We propose that in a minority of cases there is an association between vascular tumors and vascular malformations. Methods: We retrospectively identified clinical cases in our medical centers and others reported in the medical literature that demonstrate an association between vascular tumors and vascular malformation clinically or histologically. Results: We identified several clinical or histologic settings in which vascular tumors and vascular malformations were associated: (1) coexistence of hemangioma and vascular malformation at the same anatomic site or in close proximity; (2) pyogenic granuloma arising within a vascular malformation; (3) hemangioma associated with developmental vascular anomalies; (4) spindle-cell hemangioendothelioma and venous malformation; (5) kaposiform hemangioendothelioma and lymphatic malformation. Conclusion: The biologic classification of vascular birthmarks distinguishing vascular tumors from vascular malformations is clinically useful and forms the framework for our understanding of vascular anomalies. These examples suggest that in a small minority of patients there is an association between vascular tumors and vascular malformations. (J Am Acad Dermatol 2000;42:275-9.)