Bloom's syndrome (BS) is an autosomal recessive disorder that predisposes individuals to a wide range of cancers. The gene mutated in BS, BLM, encodes a member of the RecQ family of DNA helicases. The precise role played by these enzymes in the cell remains to be determined. However, genome-wide hyper-recombination is a feature of many RecQ helicase-deficient cells. In eukaryotes, a central step in homologous recombination is catalyzed by the RAD51 protein. In response to agents that induce DNA double-strand breaks, RAD51 accumulates in nuclear foci that are thought to correspond to sites of recombinational repair. Here, we report that purified BLM and human RAD51 interact in vitro and in vivo, and that residues in the N- and C-terminal domains of BLM can independently mediate this interaction. Consistent with these observations, BLM localizes to a subset of RAD51 nuclear foci in normal human cells. Moreover, the number of BLM foci and the extent to which BLM and RAD51 foci co-localize increase in response to ionizing radiation. Nevertheless, the formation of RAD51 foci does not require functional BLM. Indeed, in untreated BS cells, an abnormally high proportion of the cells contain RAD51 nuclear foci. Exogenous expression of BLM markedly reduces the fraction of cells containing RAD51 foci. The interaction between BLM and RAD51 appears to have been evolutionarily conserved since the C-terminal domain of Sgs1, the Saccharomyces cerevisiae homologue of BLM, interacts with yeast Rad51. Furthermore, genetic analysis reveals that the SGS1 and RAD51 genes are epistatic indicating that they operate in a common pathway. Potential roles for BLM in the RAD51 recombinational repair pathway are discussed.