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The tendency for breast cancer to occur more frequently on the left side than the right may reflect the effect of handedness. Handedness might influence movement on the two sides and the secretion and clearance of carcinogenic agents within the breasts. If movement is protective, the relationship between handedness and breast cancer laterality should be discordant: the disease should occur more frequently on the side of the nondominant hand. This hypothesis was tested in a sample of 1736 patients from Roswell Park Memorial Institute. The findings were consistent with the hypothesis, but the effect was small and not statistically significant. Several factors, including the low proportion of left-handed women in the American population, indicate that a much larger sample of cases is needed to adequately test the effect of handedness on breast cancer laterality.