Infantile hemangiomas are the most common tumor of infancy. Risk factors for complications and need for treatment have not been studied previously in a large prospective study. This study aims to identify clinical characteristics associated with complications and the need for therapeutic intervention. We conducted a prospective cohort study at 7 US pediatric dermatology clinics with a consecutive sample of 1058 children, aged < or = 12 years, with infantile hemangiomas enrolled between September 2002 and October 2003. A standardized questionnaire was used to collect data on each patient and each hemangioma, including clinical characteristics, complications, and treatment. Twenty-four percent of patients experienced complications related to their hemangioma(s), and 38% of our patients received some form of treatment during the study period. Hemangiomas that had complications and required treatment were larger and more likely to be located on the face. Segmental hemangiomas were 11 times more likely to experience complications and 8 times more likely to receive treatment than localized hemangiomas, even when controlled for size. Large size, facial location, and/or segmental morphology are the most important predictors of poor short-term outcomes as measured by complication and treatment rates.