Skin fibroblast-like (FL) cells from patients with Werner's syndrome (adult progeria) regularly demonstrate frequent pseudodiploidy involving variable structural rearrangements that are clonal: variegated translocation mosaicism (VTM). Ninety-two percent of 1,538 metaphases from 29 independent strains derived from five patients with Werner's syndrome demonstrated this cytogenetic abnormality. In contrast, only eight (8.4%) of 95 non-Werner's syndrome FL cell cultures demonstrated VTM: seven with low-grade VTM (approximately 5% of 300 metaphases), and one with VTM affecting 90-100% of metaphases. Unlike the cytogenetic abnormalities observed in the terminal stages of normal FL cell cultures, VTM occurs throughout the entire lifespan of Werner's syndrome cultures. Ten of the identifiable break points in 1,005 banded metaphases accounted for 27% of all definable rearrangements. Baseline sister chromatid exchanges were not increased. Cocultivation of Werner's syndrome and normal strains did not induce VTM in the normal strain. The relationship between VTM and the reduced growth potential of Werner's syndrome FL cells is not yet understood, nor is the relationship between these in vitro abnormalities and the presumptive single gene defect that causes the progeroid clinical manifestations of Werner's syndrome.