We report a case of transient rapid growth of uterine leiomyoma in a postmenopausal woman. A 58-year old woman presented with a rapidly enlarging pelvic tumor, from a 14-week to a 20-week gestational size in the one month. Transcervical needle biopsy revealed a smooth muscle tumor with 5 mitotic figures per 10 high power fields. However, surgical specimen obtained 1 month later revealed less than 1 mitotic figure per 100 high power fields. This variation may influence the final histopathological diagnosis because mitotic index is believed to be one of the most reliable histopathological parameters for differentiating between uterine leiomyoma and leiomyosarcoma.