Among the 1786 women who participated in the survey and were not excluded owing to having missing data, there were a total of 94 cases of primary breast cancer diagnosed during the follow-up period. Of 1637 right-handers, 86 (5.3%) developed breast cancer compared with 7 (7.5%) of 93 left-handers and 1 (1.8%) of 56 women who had reported being ambidextrous. Twenty-two breast cancer cases were diagnosed before the age of 51 (average age of menopause in Australia) and 72 cases were diagnosed after the age of 51 years.
After adjusting for the other variables, left-handers were more likely to develop breast cancer than right-handers, but the confidence interval was wide (HR=1.71, 95% CI 0.79–3.74). The risk of developing pre-menopausal breast cancer was lower for left-handed women compared with those who were right-handed or ambidextrous, but this was not statistically significant (HR=0.64, 95% CI 0.09–4.82). The risk of developing post-menopausal breast cancer was significantly higher for left-handers compared to right-handers, (HR=2.59, 95% CI 1.11–6.03).