Sudden unexplained death in childhood (SUDC) is rare, with a reported incidence in the United States of 1.5 deaths per 100,000 live births compared with 56 deaths per 100,000 live births for sudden infant death syndrome in 2001. The objectives of this study include a proposal for a general definition for SUDC and presentation of 36 cases of SUDC and 14 cases of sudden unexpected death in childhood. Cases were accrued through referrals or unsolicited via our Web page (http://www.sudc.org ). Our analyses tentatively suggest a SUDC profile characterized by cases being 1 to 3 years in age, predominantly male, and frequently having a personal and family history of seizures that are often associated with a fever. A history of recent minor head trauma is not uncommon. They are usually born at term as singletons and occasionally have a family history of sudden infant death syndrome or SUDC. Most are found prone, often with their face straight down into the sleep surface. Minor findings are commonly seen at postmortem examination but do not explain their deaths. Comprehensive review of the medical history and circumstances of death and performance of a complete postmortem examination including ancillary studies and extensive histologic sampling of the brain are critical in determining the cause of death in these cases of sudden unexpected childhood death. Legislation enabling research and formation of a multicenter research team is recommended to unravel the mystery of SUDC.