In a pilot study of 138 women with mastectomies that explored the consistent finding of more left breast cancer in American women, associations were found among handedness, initial breast cancer laterality, and age. To test these findings further, 889 subjects were added to the original sample. The resulting sample of 1,027 consisted of 861 right-handed, 94 ambihanded, and 72 left-handed subjects aged 21 to 84. A chi-square test showed a statistically significant association between ipsilateral breast cancer before age 45 and contralateral breast cancer after age 44 in right-handers, x2(1, n= 861) = 4.169, p<.05. The association was also significant for left-handers, x2(1 n= 72) = 4.036, p <.05. In addition, a chi-square test showed a statistically significant association between handedness and earlier development of breast cancer, x2(1, n = 933) = 13.9, p <01. Almost twice as high a percentage of left-handers as right-handers developed breast cancer before age 45. Log-linear analysis demonstrated no significant associations of nodes with either handedness or age. Overall, there was no statistically significant association between the laterality of breast cancer and a history of breast cancer in a primary relative, x2(1, n = 1,024) = 1.38, p > .05. In some presently unknown way, lateral asymmetries of neurotransmitters may contribute to an explanation of the findings.