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Cancer runs in families, suggesting a heritable component, but the contribution of environmental factors cannot be neglected. Studies on spousal risk can partly disentangle the environmental contribution but miss shared environmental factors during childhood and adolescence. Here, we examined the familial aggregation of common cancers among 80 281 Swedish-born adoptees, identified from the national Swedish Multigeneration Register, and linked them to the Swedish Cancer Registry. Standardized incidence ratios (SIRs) were calculated for common cancers (colorectal, lung, breast, prostate, and skin cancers) in the adoptees whose adoptive parents were diagnosed with concordant cancers, compared with the general population. SIRs in adoptees with an affected adoptive parent ranged from 1.00 (breast cancer) to 1.28 (skin cancer), whereas the SIRs in nonadoptees with an affected parent ranged from 1.63 (colorectal cancer) to 2.12 (skin cancer). Environmental factors account for around 0–28% of the familial aggregation. Cancer sites with high environmental contributions were observed for skin and colorectal cancers, which are known to have strong environmental causes.