Bloom's syndrome is a rare autosomal recessively transmitted disorder, the main clinical feature of which is small body size. A sun-sensitive, erythematous facial skin lesion, an excess of well-demarcated hyper- and hypopigmented skin lesions located anywhere on the body, and increased numbers of bacterial infections due to immunodeficiency are accompanying features of diagnostic value. In Bloom's syndrome, the complications are formidable: cancer, chronic lung disease, and diabetes. Cancers of the types and sites seen in the general population arise frequently and unusually early. Bloom's syndrome cells are hypermutable, and excessive numbers of somatic mutations are responsible for many of the clinical features. The clinical diagnosis is confirmed cytogenetically by demonstrating a characteristic chromosome instability.