To investigate the contribution of a founder deletion in the CHEK2 gene to the burden of breast cancer in Poland we studied 4,454 women with breast cancer and 5,496 population controls. Cases and controls were genotyped for the presence of a 5,395 bp founder deletion that removes exons 9 and 10 of the CHEK2 gene. This deletion has recently been described in a Czech and Slovak population. The cases and controls had previously been tested for two protein-truncating (IVS2 + 1G > A and 1100delC) and one missense CHEK2 mutation (I157T) which are characteristic for the population. The exons 9 and 10 deletion was present in 0.4% of the controls, in 1.0% (19 of 1,978) of unselected breast cancer cases (OR=2.2; 95% CI: 1.2-4.0; p = 0.01) and in 0.9% (28 of 3,228) of the early-onset cases (OR=2.0; 95% CI: 1.3-1.8; p = 0.02). One of the three truncating CHEK2 mutations (del5395; 1100delC or IVS2 + 1G > A) was seen in 101 of 4,454 (2.3%) cases and in 58 of 5,496 controls (1.1%) (OR=2.2; 95% CI: 1.6-3.0 p < 0.0001). A 5,395 bp founder deletion contributes to the burden of breast cancer in Poland. The deletion was present in 0.9% of the women with breast cancer diagnosed under the age of 51 and in 0.9% of women with breast cancer over the age of 50. This is one of the most common protein-truncating CHEK2 variants in Poland. Overall, 2% of all breast cancers in Poland can be attributed to one of three protein-truncating mutations in CHEK2.