Chromosomal abnormalities in cultured peripheral lymphocytes from 14 dermatomyositis patients and their first-degree relatives were examined. Fifteen normal subjects were also studied. A low increase was observed in the frequency of chromosome aberrations in patients who had not received any medical treatment but not significantly higher versus the controls; the detected chromosome aberrations were in the form of acentric fragments and dicentrics. The patients who had received medication showed an increased frequency of dicentrics, acentric fragments and chromosome breaks, although the frequency was not significantly higher than that of normal subjects. There were no statistically significant differences in the number of chromosome aberrations between men and women patients with dermatomyositis. Only one family member, (2.22%) a boy of 18 years old who suffered from the same disease like his mother, was found to have chromosomal abnormalities in peripheral blood lymphocytes. The presence of autoantibodies found in the sera of Dm patients could not be correlated with the presence of chromosomal anomalies.