Aim The CHEK2 ∗1100delC mutation confers a relative risk of two for breast cancer (BC) in the general population. This study aims to explore the excess cancer risk due to the CHEK2 ∗1100delC mutation within a familial non- BRCA1 / 2 breast cancer setting. Patients and Methods Cancer incidences were compared between first degree relatives of 107 familial breast cancer patients positive for the CHEK2 ∗1100delC mutation ( CHEK2 positive families) and first degree relatives of 314 familial breast cancer patients without the CHEK2 ∗1100delC mutation ( CHEK2 negative families). All families were derived from the same pool of familial non- BRCA1 / 2 breast cancer families ( n = 2554). Medical information of 2188 first degree relatives of these families was analysed for cancer risk. CHEK2 ∗1100delC status of relatives was unknown. Results Increased breast cancer risk (hazard ratio (HR) 2.0 (95% confidence interval (CI): 1.4–2.7), p CHEK2 ∗1100delC positive index cases compared to sisters of CHEK2 ∗1100delC negative index cases. HR was 1.6 (95% CI: 1.0–2.4) for mothers of CHEK2 positive versus negative index cases ( p = 0.041). For second primary breast cancers HR was increased in CHEK2 ∗1100delC positive index cases (HR 2.1, 95% CI: 1.3–3.3, p = 0.003) and their sisters (HR 2.6, 95% CI: 1.1–6.1, p = 0.025). Conclusion There is an excess breast cancer risk in first degree relatives of CHEK2 ∗1100delC positive non- BRCA1 / 2 familial breast cancer patients compared to non- CHEK2 ∗1100delC familial breast cancer relatives. Genotyping for the CHEK2 ∗1100delC mutation in a familial breast cancer setting contributes to optimal clinical surveillance in countries in which this mutation is prevalent. Carriers and female relatives are eligible for stringent breast surveillance programs.