Infantile hemangioma is a benign tumor of endothelial cells (ECs) that is well known to pediatricians because of its frequency and sometimes disturbingly rapid growth. The primary defect(s) that cause hemangioma have not yet been elucidated, but recent studies have revealed that hemangiomas are composed of clonal ECs that exhibit abnormal properties in vitro. These studies support the hypothesis that hemangiomas arise when a somatic mutation occurs in a single endothelial progenitor, leading to dysregulated activity of one or more genes that control EC growth. Aberrant endothelial growth may in turn alter patterns of gene expression in nearby cells--in the overlying epidermis, for example--and account for observed extrinsic alterations that may influence hemangiogenesis. Unraveling the cellular and molecular basis of hemangioma is likely to yield critical insights into the mechanisms of EC proliferation and regression that may be applicable to other cardiovascular disorders.