A １２-year-old, premenarchal girl with a rapidly growing breast lump was transferred to our hospital. Ultrasonography revealed a ７．９×７．７×３．３cm large well-circumscribed hypoechoic tumor in the medial region of her right breast. After ２ months of this first visit, the breast lump had enlarged to１０ cm in diameter and was consequently subjected to a wide excision. Size of the tumor was９．５×９．２×３．０cm. Histopathological diagnosis was fibroadenoma without any malignant component. One year after the operation, a new lump with a diameter of４cm was detected in the lateral region of her right breast that had a similar appearance of the previous tumor. The tumor was excised as earlier and diagnosed as fibroadenoma.
We present an extremely rare case of a rapidly growing breast fibroadenoma in a premenarchal girl. To the best of our knowledge, there were only ２１ cases reported in Japan. Since the patient is concerned that another asynchronous fibroadenoma might occur, she needs a long-term and careful follow-up
