Recently, the 1100delC variant of cell cycle checkpoint kinase 2 (CHEK2) has been reported to confer a colorectal cancer risk in hereditary non-polyposis-colorectal cancer (HNPCC) and HNPCC-related families in the Netherlands. To investigate whether CHEK2 mutations confer increased cancer risk in HNPCC and HNPCC-related families in Poland, we genotyped 463 probands from HNPCC and HNPCC-related families, and 5,496 controls for 4 CHEK2 alleles (1100delC, IVS2+1G>A, del5395, I157T). All 463 probands were screened for mutations in the HNPCC-related genes MSH2, MLH1 and MSH6. A positive association was observed for HNPCC-related cancer and the I157T missense CHEK2 mutation (OR = 1.7; p = 0.007), but not for the truncating alleles (OR = 1.0; p = 1.0). The association with the I157T was seen both for the 117 cases who fulfill Amsterdam criteria (OR = 1.9; p = 0.1) and for the 346 cases who do not fulfill the criteria (OR = 1.6; p = 0.03). One hundred forty-five of the 463 families had a mutation in MSH2, MLH1 or MSH6 (MMR-positive families). A positive association between the CHEK2 I157T mutation and HNPCC-related cancer was observed only for MMR-negative cases (OR = 2.1; p = 0.0004), but not for MMR-positive cases (OR = 0.8; p = 0.9). The association with I157T was particularly strong for MMR-negative cases with familial colorectal cancer (2 or more first-degree relatives affected) (OR = 2.5; p < 0.0001). We conclude that the I157T variant of CHEK2 increases the risk of colorectal cancer among MMR-negative, HNPCC/HNPCC-related families in Poland.