Cultured blood cells of an 86-year-old healthy woman displayed a chromosomal mosaic. A large proportion of the cells exhibited an identical abnormal karyotype, most likely the result of a translocation between a number 2 chromosome and a long acrocentric. The mode of origin of the aberration, and possible similarities in the developmental pattern that this mosaicism shares with leukemias and Waldenström’s macroglobulinaemia are discussed.