The CHEK2*1100delC mutation has been reported to confer a twofold increased risk of breast cancer among carriers. The frequency of the mutation varies among populations. The highest frequency has been described in Northern and Eastern European countries; the frequency may be much lower in North America. In this study, our aim was to determine the frequency of CHEK2*1100delC in members of breast cancer families who tested negative for a deleterious mutation in BRCA1/2 at the University of Michigan Comprehensive Cancer Center. We genotyped 102 members from 90 families for CHEK2*1100delC. Most of these families had several cases of breast cancer or ovarian cancer (or both), as well as multiple members with other cancer types in a single lineage. No CHEK2*1100delC mutations were detected in any of the 102 individuals, including 51 women diagnosed with breast cancer at an early age (<45 years), 8 women with bilateral breast cancer, 3 men with breast cancer, and 8 women with ovarian cancer. Our data are consistent with the reported very low frequency of CHEK2*1100delC mutations in North American populations (compared with Northern Europe), rendering CHEK2*1100delC such an unlikely culprit in BRCA1/2 negative families that routine testing of these families appears unwarranted.