Specific chromosomal abnormalities, e.g. del(7q), t(12;14), 12 trisomy, and the rearrangement of 6p, are seen in approximately 30% of uterine leiomyomas. We investigated the association between the shrinkage effect of GnRH agonist on uterine leiomyomas and t(12;14), the second most frequent chromosomal abnormality in myomas. This study involved 42 women with uterine leiomyomas treated with a gonadotropin releasing hormone (GnRH) agonist before surgery. The volume of the largest myoma nodule was measured by MRI before and at 12 weeks after the beginning of GnRH agonist treatment, and the percentage change in volume was calculated. A specific chromosomal abnormality, t(12;14), was examined on thin sections of frozen leiomyomas by fluorescence in situ hybridization with chromosome-specific probes. Of the 42 tumors, 8 (19%) showed translocation. The mean (+/- SD) percentages change in volume of the largest myomas without and with translocation were -32+/-24 and 23+/-60%, respectively (p=0.006). The myomas showing translocation had significantly less reduction in size with GnRH treatment than did those without translocation. No myoma with trisomy 12 was found. On the basis of our results, we assumed that uterine leiomyomas showing t(12;14) are not so dependent on ovarian hormones for growth.