Chromosome analysis of short-term cultures from uterine leiomyomas revealed trisomy 12 in six cases. In four of the tumors + 12 was the only karyotypic change. The fifth case also had trisomy 4, whereas the sixth had loss of one X chromosome as an additional aberration. These findings indicate that trisomy 12 characterizes a new cytogenetic subgroup of uterine leiomyoma.