The Bloom's Syndrome Registry is comprised of most of the bona fide cases of Bloom's syndrome (BS) that have been recognized since 1954, when the condition first was described. The Registry was developed, and its maintenance in the coming years is proposed, primarily because of one particular clinical feature of the syndrome, to wit, cancer proneness. The Registry has become a vehicle for the surveillance of the world's BS population with respect to the incidence, age of onset, and distribution of types of neoplasia, in both affected homozygotes and in heterozygotes (carriers of the BS gene, who appear phenotypically normal). Already the data suggest that of the known recessively-transmitted genetic disorders that predispose to cancer BS does so most strongly. Furthermore, the distribution of types of cancers may be similar to that in the general population. Simultaneously, information on the natural course of BS (interesting in several ways) is being accumulated. Also, the family contacts provide a source of specimens, such as blood cells, tissue culture cell lines, and tumor DNA for use in other studies. In one respect, then, the Registry, in being a central repository for information on a affected families throughout the world, may be viewed as a research resource. Support is requested not for a research project in the usual sense but for maintenance of the Bloom's Syndrome Registry itself, i.e., to monitor a cancer-prone population, one that is small but unusually interesting for several disciplines (e.g., oncology, immunology, cytogenetics, and DNA metabolism).