The biologically based hypothesis that magnetic fields increase the risk of conditions related to cardiac arrhythmia and acute myocardial infarction but not chronic cardiovascular disease was initially supported by the results of an epidemiologic study. High rates of cardiovascular disease and relatively common exposure to magnetic fields made it an important public health question. Most of the epidemiologic studies that followed showed no effect. In this paper the basis for both this hypothesis and the epidemiologic studies that tested it are presented. It was concluded that the evidence speaks against an etiologic relation between exposure to electric and magnetic fields and cardiovascular disease. This effort represents an interesting case study of a scientific inquiry that has been successfully resolved despite numerous methodological difficulties inherent in research on low-level environmental exposures.