Mucinous carcinoma arising in a background of fibroadenoma is rare. We report a case of a 34 year old woman who presented in our hospital with a mobile mass in the right breast. Physical examination, breast ultrasound and fine needle aspiration cytology (FNAC) were all concordant for diagnosis of fibroadenoma. However microscopic examination of excisional biopsy revealed mucinous carcinoma within a background of fibroadenoma. The patient underwent right mastectomy with dissection of the right axillary lymph nodes. Nodal metastasis was not seen. She was also treated with chemotherapy and radiotherapy and has shown no sign of recurrence for more than one year since after surgery. This report highlights one of the pitfalls of FNAC, particularly in the diagnosis of mixed lesions of the breast.