In this study we determined in what proportion of consecutive malignant melanoma (MM) cases the 657del5 mutation of exon 6 of the NBS1 gene can be detected and whether it is associated with the occurrence of MM. Two groups of patients were studied: a series of 80 consecutive patients with histologically confirmed MM of the skin diagnosed in the city of Szczecin, Poland, and a series of 530 consecutive individuals selected at random by family doctors from the city of Szczecin. Molecular examination included an allele-specific polymerase chain reaction assay for the NBS1 founder mutation (657del5), genomic sequencing, loss of heterozygosity analysis using CA-repeat microsatellite markers, and haplotype analysis. The NBS1 founder mutation was detected in two of the 80 (2.5%) MM cases and in three of the 530 individuals (0.6%) from the general population. The difference was not statistically significant. However, examination of tumorous DNA from the patients with MM and NBS1 mutation revealed loss of heterozygosity in both cases. Haplotype analysis revealed that allellic loss affects wild-type alleles. Breast cancer was found in second-degree relatives of both MM probands with NBS1 mutations. One of these probands was simultaneously affected with breast cancer. It seems that the 657del5 mutation of exon 6 of NBS1 gene may be responsible for the occurrence of a small proportion of MM patients, characterized by the occurrence of breast cancer among their relatives.