Forty-nine specimens from a variety of vascular lesions were analyzed for cellular characteristics. Two major categories of lesions emerged from this investigation: hemangiomas and vascular malformations. This classification and its implications are justified by several considerations. Hemangiomas in the proliferating phase (n = 14) were distinguished by (1) endothelial hyperplasia with incorporation of [3H]thymidine, (2) multilaminated basement membrane formation beneath the endothelium, and (3) clinical history of rapid growth during early infancy. Hemangiomas in the involuting phase (n = 12) exhibited (1) histologic fibrosis and fat deposition, (2) low to absent [3H]thymidine labeling of endothelial cells, and (3) rapid growth and subsequent regression. The endothelium in hemangiomas had many characteristics of differentiation: Weibel-Palade bodies, alkaline phosphatase, and factor VIII production. Vascular malformations (n = 23) demonstrated no tritiated thymidine incorporation and normal ultrastructural characteristics. These lesions were usually noted at birth, grew proportionately with the child, and consisted of abnormal, often combined, capillary, arterial, venous, and lymphatic vascular elements. This cell-oriented analysis provides a simple yet comprehensive classification of vascular lesions of infancy and childhood and serves as a guide for diagnosis, management, and further research.