Hemangiomas are the most common type of tumor in infants. As they are endothelial cell–derived neoplasias, their growth can be regulated by the autocrine-acting Tie2 ligand angiopoietin 2 (Ang2). Using an experimental model of human hemangiomas, in which polyoma middle T–transformed brain endothelial (bEnd) cells are grafted subcutaneously into nude mice, we compared hemangioma growth originating from bEnd cells derived from wild-type, Ang2+/–, and Ang2–/– mice. Surprisingly, Ang2-deficient bEnd cells formed endothelial tumors that grew rapidly and were devoid of the typical cavernous architecture of slow-growing Ang2-expressing hemangiomas, while Ang2+/– cells were greatly impaired in their in vivo growth. Gene array analysis identified a strong downregulation of NADPH oxidase 4 (Nox4) in Ang2+/– cells. Correspondingly, lentiviral silencing of Nox4 in an Ang2-sufficient bEnd cell line decreased Ang2 mRNA levels and greatly impaired hemangioma growth in vivo. Using a structure-based approach, we identified fulvenes as what we believe to be a novel class of Nox inhibitors. We therefore produced and began the initial characterization of fulvenes as potential Nox inhibitors, finding that fulvene-5 efficiently inhibited Nox activity in vitro and potently inhibited hemangioma growth in vivo. In conclusion, the present study establishes Nox4 as a critical regulator of hemangioma growth and identifies fulvenes as a potential class of candidate inhibitor to therapeutically interfere with Nox function.