Uterine leiomyomas are the most frequently diagnosed tumors of the female genital tract and the primary cause of hysterectomy in premenopausal women in the United States. In vitro model systems for studying these tumors are limited, due to poor culture growth. For the present study, myometrial (UtSMC) and uterine leiomyoma (UtLM) cell lines and their human telomerase reverse transcriptase (hTERT) immortalized counterparts were evaluated by GTL-banding and spectral karyotyping. UtSMC, at passage 9 showed the normal female karyotype, 46,XX. UtSMC-hTERT at passage 13 (population doubling [PD] 22 post immortalization) had two cell clones: 46,XX,der(13)t(10;13)(q11.2;p13)]/46,XX. UtLM, at passage 14, and the immortalized counterpart UtLM-hTERT (passage 13, PD 20 post immortalization), had the reciprocal translocation t(12;14)(q14;q23) in all cells and monosomy X in a fraction of cells. UtLM also displayed genomic instability with 15% of cells showing structural abnormalities involving chromosome arms 1p and 5q. UtLM-hTERT was karyotypically more stable than the parental line, thereby reflecting the inhibition of the accumulation of cytogenetic abnormalities by the maintenance of telomere integrity. This phenomenon was not observed in the UtSMC cells.