In extension of a previous study, spontaneous and clastogen-induced chromosome damage was analyzed in cultures of peripheral blood lymphocytes from six further patients with Werner syndrome (WS) and six healthy controls. In addition, sister chromatid exchange (SCE) was estimated in four of these cases. Lymphocytes of patients with various other diseases were used for another series of control experiments. Diepoxybutane (DEB), 4-nitroquinoline-1-oxide (NQO), and bleomycin (BLM) were the standard clastogens throughout the study. While the spontaneous frequency of chromosomal breakage was significantly higher in lymphocytes from all the patients than in the control cells, the basis SCE rate was unaffected in WS cells. Sensitivity of WS cells to the chromosome-damaging action of BLM did not differ from that of control cells, and their sensitivity to DEB was slightly greater than that of control lymphocytes. However, NQO induced a more distinct increase of both break and interchange aberrations in the WS cells than in control cells or cells from patients with other diseases. This effect was not found for the SCE rate. Our data demonstrate the exceptional cytogenetic features of this syndrome: Although the spontaneous and the DEB- and NQO-induced chromosomal breakage rate would suggest that WS is like a classic chromosomal instability syndromes, the lack of sensitivity of WS cells to bleomycin and their stable SCE frequency compared with that of control cells clearly delimitate this syndrome from other entities.