Mucious cystic neoplasm of pancreas is a cystic neoplasm composed of columnar, mucin-producing epithelium and is supported by ovarian-type stroma. The key to the cytologic evaluation of pancreatic cystic lesions is to recognize the cytologic components as being diagnostic of a mucin-producing cystic neoplasm, as all of these neoplasms need to be resected. We report the use of fine needle aspiration cytology in the diagnosis of an invasive mucinous cystic carcinoma confirmed by partial pancreatectomy. The cytologic specimen showed a abundant mucin background and sheets or papillae of neoplastic cells. There are mucin-containing columnar cells that show a variable degree of cytologic atypia.