We examined potential confounding factors that, if unaccounted for, could possibly produce a spurious association in a study of breast cancer among women occupationally exposed to magnetic fields. For each risk factor, we estimated strength of association, prevalence in the general population, and prevalence of the risk factor in the exposed group required to explain completely hypothetical odds ratios between occupational exposure to magnetic fields and breast cancer. We performed similar analyses for two, three, four, and five confounding factors acting simultaneously. Factors numerically capable of substantial confounding included obesity, continent of birth, family history of breast cancer in a first-degree relative, densities on the mammogram, benign proliferative breast disease, history of cancer in one breast, and consumption of at least two alcoholic drinks per day. Nevertheless, only continent of birth, history of cancer, obesity, and consumption of alcohol could potentially be related to occupation. Uncontrolled confounders, either alone or in combination, could possibly account for odds ratios in the 1.2-1.3 range but were very unlikely to produce an odds ratio of more than 1.5. A spurious negative association between magnetic fields and breast cancer could occur if the exposed group included a large number of immigrants from Asia and Africa.