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To the Editor: Dr McTiernan and colleagues¹ of the Women's Health Initiative (WHI) reported that increased physical activity was associated with a 14% decrease in the risk of breast cancer in postmenopausal women. These results were not based on a randomized trial of exercise vs no exercise, but rather were from observational data. An earlier randomized WHI study² of estrogen plus progesterone therapy vs placebo demonstrated that the prior observational studies that had demonstrated a cardiovascular benefit for hormone therapy were incorrect. It was clear that there was a strong bias in the prior observational studies toward showing a cardiovascular benefit in the women who had selected the hormone replacement therapy intervention. Thus, it is possible that a similar bias occurred in this study as well.

Harry B. Burke, MD, PhD
Department of Medicine
George Washington University School of Medicine
Washington, DC

1. McTiernan A, Kooperberg C, White E, et al. Recreational physical activity and the risk of breast cancer in postmenopausal women: the Women's Health Initiative Cohort Study. JAMA. 2003;290:1331-1336. FREE FULL TEXT 2. Rossouw JE, Anderson GL, Prentice RL, et al, Writing Group for the Women's Health Initiative Investigators. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results from the Women's Health Initiative randomized controlled trial. JAMA. 2002;288:321-333. FREE FULL TEXT

Letters Section Editor: Stephen J. Lurie, MD, PhD, Senior Editor.

JAMA. 2003;290:3193.

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