12,178 women were questioned about their reproductive history, demography, lifestyle and innate hand preference, and their answers were recorded to provide a "baseline". The same women were interviewed 16 years later, and the data was used to calculate total person-years lived in the sample. The sample comprised 11.6 % left-handed women. Mean age at baseline was similar for left and non-left-handed women: 47.4 and 47.0 years respectively. These groups did not differ in anthropometry, socioeconomic status, smoking habits or family history of breast cancer. The only difference was in child bearing: 21.8 % of left-handed versus 10.9 % of non-left-handed women had borne no children. 426 women from the whole group had breast cancer. The non-left-handed group was taken as the reference group. The relative risk for breast cancer in left-handed women compared with non-left-handed women was calculated and was 1.39 overall (see table).