Exposures that occur in utero or during childhood have long been the cornerstone of research examining the effects of environmental agents, since these groups are considered susceptible subpopulations most likely to be impacted by toxic physical or chemical agents. Since the first reports suggesting a linkage between residential proximity to power lines and childhood cancer appeared in 1979,1 an ongoing scientific and public debate2 has raged over the possible health effects of power-frequency (50-60 Hz) magnetic fields (MFs). Despite years of effort, a special federal funding emphasis,3- 4 heated discussions, name-calling,5- 6 and some innovative research, no scientific consensus has emerged on the topic. Epidemiologists have honed their methods and examined a vast array of alternative hypotheses; some physicists declared that such effects are theoretically impossible7; and biologists continue to search for a consistent and reproducible cellular or animal model, while other investigators have cataloged the wide array of biological effects associated with exposure to power- or radio-frequency fields.8 Comprehensive scientific reviews conducted by various agencies, including the World Health Organization, the World Health Organization International Agency for Research on Cancer, the Health Protection Agency National Radiological Protection Board, and the National Institutes of Health National Institute of Environmental Health Sciences,9- 12 all have found that power-frequency MFs may play a role in childhood cancer and possibly other childhood disorders but were unable to establish a mechanism or animal model to definitively support their findings. The debate for more than 2 decades has hinged on the persistent observation that prolonged exposure of children to MFs higher than a threshold of about 4 milligauss is associated with an approximate 2-fold elevation in leukemia risk. Since the earliest reports, we have learned a great deal about improving MF exposure assessment and the application of study designs to address possible selection biases or confounders. Yet the underlying biological basis for this association has not advanced that much, partly because much scientific scrutiny has focused on testing alternative hypotheses or examining other potential explanations. Perhaps the biology needs a closer look.