As of 1996 the 100th cancer was diagnosed in Bloom's syndrome. The cancers have been regularly documented since 1960 in a program of surveillance referred to as the Bloom's Syndrome Registry. Tabulated here are their types and ages of onset. The 100 cancers arose in 71 of the 168 registered individuals. Represented in Bloom's syndrome are both the cancers that commonly affect the general population and the rare tumors of early childhood. This body of information has become sufficiently large to be useful to geneticists and physicians in advising affected families concerning cancer risk. Of more general significance, however, the distribution of cancer sites and types sets Bloom's syndrome apart from other cancer-predisposing genetically determined conditions, affirming its experimental value as a model for analyzing the nonenvironmental component in the etiology of the generality of human cancer.