The lifespan of fibroblasts from genetic syndromes with reduced DNA repair or chromosome stability has been measured. Cells from Bloom’s syndrome, Cockayne’s syndrome, Fanconi’s anaemia and 2 out of 3 cases of ataxia telangiectasia had a significantly reduced growth potential in comparison to controls. In each case the longevity of several parallel populations was measured and the greatest variability in lifespan was observed with Cockayne’s syndrome cells. The fibroblasts from 1 ataxia telangiectasia patient and a Friedreich’s ataxia patient grew to the passage levels seen in control cultures. The results suggest that repair processes are necessary for cells to achieve their maximum in vitro lifespan, and support the error theory rather than the programme theory of ageing.