This study was conducted to define the cytogenetically critical regions of uterine leiomyomata, hoping to demonstrate the presence of possible genes involved in their evolution. It was carried out on 25 randomly selected uterine leiomyoma specimens obtained from 16 patients during hysterectomy or myomectomy operations. Successful tissue culture and karyotyping were performed in 19 specimens. There was no correlation between patient age, gravidity, or presenting symptom and the presence of chromosomal abnormality. A significant correlation was found between short culture turnaround time and the occurrence of chromosomal abnormality. Abnormal clonal karyotypes were present in 6 specimens, non-clonal abnormalities in 4 specimens and normal karyotypes were found in 9 specimens. Myomas with cross section > 4 cm showed an increased incidence of abnormal karyotypes and a statistically significant higher incidence of clonal abnormalities. On the other hand, submucous myomas presented fewer clonal abnormalities than did intramural or subserosal myomas. Clonal chromosomal abnormalities involved 5 different chromosomes (2, 7, 8, 12, 22), which indicate genetic heterogeneity of such benign tumors and the need of molecular cytogenetic studies or molecular studies to characterize possible candidate genes at specific chromosomal breakpoints.