More than 13,000 children are estimated to be newly diagnosed with cancer each year in the United States (1). Despite substantial improvements in survival over the last several decades, cancer is still the leading cause of death due to disease in children ages 1 to 14 years (2). The overall incidence rate for childhood cancer has increased significantly since the 1970s, and our recent analysis of Surveillance, Epidemiology, and End Results program data for the period 1992 to 2004 points to a continuing increase (3). As we investigate potential causes, it is important to consider what we know thus far.

Gestation is a critical window of risk for most childhood cancers. In utero initiation is supported circumstantially by the young age at diagnosis (4) as well as the histologic and cytogenetic resemblance of embryonal tumor (5) and leukemia (6,...

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