Skin fibroblast-like (FL) cells from patients with Werner’s syndrome (adult progeria) demonstrate multiple, stable, structural chromosome rearrangements (variegated translocation mosaicism) which can be used to identify cytogenetically marked clones of cells within a mass culture. We have cytogenetically followed eight FL cell strains (from two patients) throughout their entire in vitro replicative lifespans, and we show a correlation between the expansion and attenuation of individual clones and the growth of the mass cultures. One strain, which was aged several times, demonstrated a generally reproducible pattern of clonal succession but, surprisingly, also demonstrated, in two parallel derivative cultures, the late emergence of two relatively rapidly growing clones that had not been observed in the parental culture. These observations suggest that clonal succession and clonal attenuation occur in mass cultures, as had been predicted on the basis of dilute-plating cloning experiments. Our results may have implications for models of in vitro cellular senescence. In addition, there are interesting parallels with the tissue hyperplasia associated with in vivo aging, and this observation is compatible with the suggestion that skin FL cells in vitro provide a model for hyperplasia in viv