A cohort of 118 twin pairs with infantile hemangioma was reviewed to determine clinical characteristics and to estimate the relative contribution of hereditary and/or environmental factors to pathogenesis. Questionnaires were sent to parents of twins in Massachusetts and Australia. Zygosity was determined by clinical questions. Accuracy of ascertainment was based on clinical and anatomic criteria. Race, gender ratio, topographic distribution, number of lesions, age of onset, and complications were all consistent with demographic data from other studies of infantile hemangioma. Of the 118 pairs, 40 were monozygotic twins (32 like-sex females and 8 like-sex males) and 78 were dizygotic (30 like-sex females, 17 like-sex males, and 31 unlike sex). The concordances in monozygotic and dizygotic female twins were 32% and 20%, respectively (p= 0.5). In like-sex male monozygotic and dizygotic pairs, the concordances were 25% and 12%, respectively (p= 0.4). These differences were not statistically significant. This study suggests that hereditary factors are not paramount in causing hemangioma. However, being female, fair-skinned, Caucasian, and having a positive family history presumably lowers the threshold for hemangiogenesis wherein extragenetic factors can trigger the appearance of this most common tumor of infancy.