Breast cancer risk in male twins: joint analyses of four twin cohorts in Denmark, Finland, Sweden and the United States

To test the hypothesis that in utero exposure to high levels of oestrogen i ncreases the risk of male breast cancer, we followed 115 235 male twins for more than 3.5 million person-years at risk. We observed 11 cases of male b reast cancer versus 16.16 expected based on national rates (standardized ra te ratio 0.68, 95% confidence interval 0.34-1.22) and conclude that any adv erse influence of in utero oestrogen exposure is likely to be small. (C) 20 00 Cancer Research Campaign.