The chromosomes of four cell lines derived from skin biopsies of four patients with xeroderma pigmentosum (XP) were studied and compared with corresponding controls. In the early passages, all XP lines had normal chromosome constitution with rates of polyploidy and chromosome aberrations within the range found in the controls. In the later passages, the XP cell lines had higher levels of chromosome abnormalities. The degree of abnormalities varied greatly among the XP lines; two lines had very high level of polyploidy (up to 50%) and cells with chromosome aberrations (up to 79%), the other two XP lines had a normal level of polyploidy and a slightly higher incidence of cells with chromosome aberrations than normal. The most common type of aberration was dicentrics.