PURPOSE: CHEK2*1100delC heterozygosity may be associated with an increased risk of breast cancer; however, it is unclear whether the evidence is sufficient to recommend genotyping in clinical practice. PATIENTS AND METHODS: We identified studies on CHEK2*1100delC heterozygosity and the risk of unselected, early-onset, and familial breast cancer through comprehensive, computer-based searches of PubMed, EMBASE, and Web of Science. Aggregated risk estimates were compared with previous estimates for BRCA1 and BRCA2 mutation heterozygotes. RESULTS: By using fixed-effect models for CHEK2*1100delC heterozygotes versus noncarriers, we found aggregated odds ratios of 2.7 (95% CI, 2.1 to 3.4) for unselected breast cancer, 2.6 (95% CI, 1.3 to 5.5) for early-onset breast cancer, and 4.8 (95% CI, 3.3 to 7.2) for familial breast cancer. For familial breast cancer, this corresponds to a cumulative risk of breast cancer at age 70 years in CHEK2*1100delC heterozygotes of 37% (95% CI, 26% to 56%), which compares with similar previous estimates of 57% (95% CI, 47% to 66%) for BRCA1 mutation heterozygotes and 49% (95% CI, 40% to 57%) for BRCA2 mutation heterozygotes. CONCLUSION: These meta-analyses emphasize that CHEK2*1100delC is an important breast cancer-predisposing gene, which increases the risk three- to five-fold. Because the cumulative risk of breast cancer at age 70 years among familial patient cases for CHEK2*1100delC heterozygotes is almost as high as that for BRCA1 and BRCA2 mutation heterozygotes, genotyping for CHEK2*1100delC should be considered together with BRCA1 and BRCA2 mutation screening in women with a family history of breast cancer.