ARTICLE ABSTRACTBackground:Paternal transmission of a BRCA mutation has been reported to increase the risk of breast cancer in offspring more than when the mutation is maternally inherited. As this effect might be caused by referral bias, the aim of this study was to assess the parent-of-origin effect of the BRCA1/2 mutation on the breast cancer lifetime risk, when adjusted for referral bias.Methods: A Dutch national cohort including 1,314 proven BRCA1/2 mutation carriers and covering 54,752 person years. Data were collected by family cancer clinics, via questionnaires and from the national Dutch Cancer Registry. The parent-of-origin effect was assessed using Cox regression analyses, both unadjusted and adjusted for referral bias. Referral bias was operationalized by number of relatives with cancer and by personal cancer history.Results: The mutation was of paternal origin in 330 (42%, P < 0.001) BRCA1 and 222 (42%, P < 0.001) BRCA2 carriers. Paternal origin increased the risk of prevalent breast cancer for BRCA1 [HR, 1.54; 95% confidence interval (CI), 1.19–2.00] and BRCA2 carriers (HR, 1.40; 95% CI, 0.95–2.06). Adjusted for referral bias by several family history factors, these HRs ranged from 1.41 to 1.83 in BRCA1 carriers and 1.27 to 1.62 in BRCA2 carriers. Adjusted for referral bias by personal history, these HRs were 0.66 (95% CI, 0.25–1.71) and 1.14 (95% CI, 0.42–3.15), respectively.Conclusion: A parent-of-origin effect is present after correction for referral bias by family history, but correction for the personal cancer history made the effect disappear.Impact: There is no conclusive evidence regarding incorporating a BRCA1/2 parent-of-origin effect in breast cancer risk prediction models. Cancer Epidemiol Biomarkers Prev; 25(8); 1251–8. ©2016 AACR.