The 1100delC germline mutation of the CHEK2 gene appears to contribute significantly to the overall breast cancer incidence in some West and North European countries, but seems to be much less frequent among breast cancer patients from other regions of Europe. In the present study we found, respectively, 3/487, 1/296 and 0/279 carriers of this mutation among breast cancer patients from the East-Central, South-East and West-Central regions of Poland. Two carriers of the 1100delC mutation were found among 120 patients with bilateral breast cancer, but only one had a previous family incidence of breast cancer. We found no carriers among 182 patients with unilateral breast cancer with family history of this tumor and among 64 patients with breast cancer and a second primary tumor at an other site. We conclude that the 1100delC mutation of the CHEK2 gene contributes little to the overall breast cancer burden in Poland, including familial cases of this malignancy. Further studies are still needed to evaluate the contribution of this mutation to the development of bilateral breast tumors.