We report on a 21-month-old white boy with the Rothmund-Thomson syndrome. The karyotype on fibroblasts from an area of skin with poikiloderma showed the 46,XY,17 + der(17),t(2;17)(q11;p13) pattern. Karyotype on fibroblasts from normal skin showed two different abnormal patterns: 47,XY, + 8 and 47,XY, + i(2q). His lymphocytes had a normal 46,XY pattern. These findings indicate in vitro abnormalities. They are explained by a degree of chromosomal instability.