Cytogenetic investigation on 19 benign tumors of uterine smooth muscle was carried out after short-term cultures. Clonal chromosome abnormalities were present in four cases; 15 had normal karyotypes. All four cases with abnormal karyotype showed changes of chromosome 12. In three cases, chromosome 12 was involved in structural rearrangements with chromosome X, 1, and 4, respectively. Breakpoints on this chromosome occurred in different regions. The fourth case had trisomy 12 and an extra deleted chromosome 2, del(2)(p22). These findings confirm the involvement of chromosome 12 in uterine leiomyoma.