A prospective epidemiological study —the first of its kind— has failed to find an association between a woman’s melatonin level and her risk of developing breast cancer. Ruth Travis and coworkers at the University of Oxford in the U.K. report in the March 17 issue of the Journal of the National Cancer Institute that, while they cannot rule out a “moderate” association, their results are a setback for the hypothesis that “endogenous melatonin concentration is a major factor in breast cancer etiology. ”