Final answer:
Needle aspiration is both a diagnostic and therapeutic procedure in fibrocystic breast disease.
Step-by-step explanation:
The procedure that is both diagnostic and therapeutic in fibrocystic breast disease is needle aspiration. Needle aspiration involves inserting a thin needle into the breast to drain fluid from a cyst or to remove cells for further examination. It is a diagnostic procedure because it can help determine the nature of a breast lump or cyst, and it is therapeutic because it can relieve pain or discomfort caused by the cyst.