BACKGROUND: Infantile hemangioma (IH) is the most common benign tumor of infancy, yet its pathogenesis is poorly understood. Notch family members are known to play a role in vascular development during embryogenesis and postnatal tumor angiogenesis, yet the role of Notch signaling in the pathogenesis of IH has not been investigated. This study aims to survey Notch expression in IH. MATERIALS AND METHODS: RNA from resected hemangioma tissue and hemangioma-derived stem cells (HemSCs) and endothelial cells (HemECs) was used for gene expression analyses by real-time PCR. Results were confirmed with immunofluorescence for protein expression in tissue. RESULTS: Real-time PCR showed that Notch family gene expression in IH is distinct from placenta and skin. Notch3 is expressed in HemSCs, but not in HemECs, indicating Notch3 is downregulated as HemSCs differentiate into HemECs. Moreover, expression of endothelial-associated Notch proteins, Notch1, Notch4, and Jagged-1 are increased in involuting hemangiomas and HemECs, suggesting that as hemangioma progresses toward involution, it acquires more differentiated endothelium. A subset of cells stained double positive for Notch3 and CD31, pointing to a potential intermediate between the HemSC cellular differentiation into HemEC. CONCLUSION: HemSCs have distinct Notch expression patterns from differentiated HemECs and from normal human endothelial cells. Notch3 is expressed in HemSCs, while Notch1, Notch4, and Jagged-1 have higher expression levels in HemECs. Notch3 was localized to the interstitial cells outside of the nascent vascular channels in proliferating IH tissue sections, but became more apparent in the perivascular cells in involuting IH. In summary, the pattern of Notch gene expression mirrors the progression from immature cells to endothelial-lined vascular channels (i.e., endothelial differentiation) that characterizes the growth and involution of IH.