['Familial clustering of prostate, breast and colorectal cancer is well established, but the familial risk of these cancers has not been determined among adoptees. The aim was to disentangle the contributions of genetic and environmental factors to the familial transmission of prostate, breast and colorectal cancer.', 'The Swedish Multi-Generation Register was used to follow all adoptees born between 1932 and 1969 (n=70,965) for prostate, breast and colorectal cancer from January 1958 up to December 2010. The risk of prostate, breast and colorectal cancer was estimated in adoptees with at least one biological parent with the same cancer type compared with adoptees without a biological parent with the same cancer type. The risk of cancer was also determined in adoptees with at least one adoptive parent with cancer compared with adoptees with an adoptive parent without cancer.', 'Adoptees with at least one biological parent with prostate, breast or colorectal cancer were more likely to have cancer of the same type than adoptees with biological parents not affected by these respective cancer types (standardised incidence ratio=SIR: 1.8 [95% confidence interval 1.2-2.7], 2.0 [1.6-2.5] and 1.9 [1.2-2.9], respectively). In contrast, adoptees with at least one adoptive parent with prostate, breast or colorectal cancer were not at an increased risk of these respective cancer types (SIR=1.2 [0.94-1.6], 0.97 [0.71-1.3], and 1.1 [0.71-1.5], respectively).'] Is familial transmission of prostate , breast and colorectal cancer in adoptees related to cancer in biological but not in adoptive parents : a nationwide family study?