Hemangioma is the most common tumor of infancy. This vascular tumor is characterized by an initial rapid proliferation followed by an inevitable regression. The life cycle of hemangioma is divided into proliferative, involuting, and involuted phases. The cellular and molecular mechanisms responsible for controlling the biological behavior of hemangioma are largely unknown. Differential display analysis using mRNA isolated from biopsy specimens representative of the 3 different phases showed increased expression of clusterin/apoJ (clust/apoJ) in the involuting samples. Clust/apoJ is a multifunctional glycoprotein that has been associated with apoptosis. Reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry showed that both the transcription and protein expression of clust/apoJ were increased in hemangioma as the tumor progressed from the proliferative to the involuting and involuted phases. This suggests that clust/apoJ is involved in regulating apoptosis during the spontaneous regression of hemangioma. It has been suggested that mast cells (MC) play a role in the regression of hemangioma. The increase in the number and proportion of clust/ apoJ-positive MC with progression of hemangioma, along with the localization of clust/apoJ to MC granules, supports this hypothesis. We suggest that MC may be synthesizing/releasing this apoptotic modulator, leading to the regression of the tumor. Better understanding of the pathogenesis of hemangioma by identification of the relevant factors involved in its regression such as clust/apoJ will result in the development of novel therapies for this condition and tumors that do not undergo spontaneous regression.