Structural chromosome aberrations were analyzed in peripheral lymphocytes of eight monozygotic (MZ) and seven dizygotic (DZ) pairs of male twins. There was no significant intrapair difference in the variance of aberration frequencies among the MZ and DZ twins. Thus, there was no evidence of a major genetic influence on the development of structural chromosome aberrations. Although a genetic component could not be excluded, it was concluded that any chromosome aberrations observed were probably due mainly to environmental influences.