Short-term cultures of samples from eight prophylactic mastectomies from five unrelated women who were genetically predisposed to breast cancer were analyzed cytogenetically. Clonal chromosome abnormalities were detected in five breasts. Three samples from two women had aberrations involving the short arm of chromosome 3, with a breakpoint in 3p14 in common. Three samples from three women had rearrangements of 1q. Two of them, one of which also displayed a 3p14 rearrangement, shared a breakpoint in 1q41. Both 1q41 and, in particular, 3p14 have been reported to be rearranged frequently in malignant breast proliferations. Whether alterations of genes in these bands are essential in mammary tumorigenesis and, if so, whether they are equally important in sporadic and in hereditary cases remains to be explored.