The Bloom's Syndrome Registry comprises the 71 individuals in whom this rare genetic disorder has been recognized between the time it was described in 1954 and the end of 1976. The major objective of the Registry is surveillance for cancer in both affected homozygotes and heterozygotes. Of the 61 homozygotes known to have had Bloom's syndrome before cancer was diagnosed and for whom follow-up has been possible, one in nine has developed cancer. Thirteen cancers have been diagnosed, in 12 individuals. The mean age in 1976 of the living individuals with the syndrome was 16.4 years. The mean age at the time cancer was diagnosed was 20 years. Cancers have been of multiple types and have affected various sites.