Abstract
A new method for analyzing concordance for a binary variable in extended pedigrees was developed to study tumor laterality in families with high incidence of breast cancer. It was found that related breast cancer patients In 15 high-risk Midwestern US families did not have their tumors on the same side more frequently than would be expected by chance. This finding is in contrast to previous studies in London and Denmark that reported a significant concordance for tumor laterality in related breast cancer patients. This result may reflect an Increasing Incidence of sporadic breast cancer in genetically susceptible families or, alternatively, Independent determination of cancer susceptibility and tumor laterality.