We have determined the mitotic stability of micro- and mini-satellite DNA sequences in SV40-immortalized Werner syndrome (WS) and control fibroblast cell lines. Five microsatellite loci were genotyped in two WS and two control SV40-immortalized fibroblast cell lines and in 154 independent primary or secondary clones derived from these. We used four minisatellite "core" or individual locus probes in Southern blot hybridization analyses to assess minisatellite stability in WS and control clones. Microsatellite allele length was stably maintained in both WS and control cells, and an upper limit for the generation of new allele lengths was estimated to be < or = 4.5 x 10(-4)/allele/generation (or < or = 2.25 x 10(-5)/CA repeat/generation). In contrast to length stability, loss of heterozygosity (LOH) at microsatellite loci ranged up to 76% at the 13 informative locus:cell line combinations. An unexpected, and counterintuitive, finding was a much lower frequency of LOH in WS than in control clones at microsatellite loci on three different chromosomes. Minisatellite band alterations (gains, losses, or band intensity differences) were 4-fold lower in WS than in control cells. Our results suggest that the chromosomal and molecular genetic instability displayed by WS cells is unlikely to be the result of a micro- or mini-satellite destabilizing defect. A second, unexpected conclusion is that WS cells may possess a novel means of either suppressing or masking LOH events in the presence of constitutional cytogenetic and molecular genetic instability.