Mucinous adenocarcinoma of the submandibular gland. A rare tumor not easily classifiable among published histologic categories for salivary gland tumors is reported. The neoplasm developed within the submandibular gland of a 78-year-old woman with invasion of the mandible and metastasis to regional lymph nodes. Histopathologically, cuboidal cells possessing clear cytoplasm and displaced round nuclei proliferated and exhibited an adenomatous pattern. Many cystic spaces surrounded by tumor cell strands were seen, mucus substance filled in the cystic spaces, and the tumor cells seemed mucus-secreting, but neither epidermoid cells nor papillary appearance could be observed. Electromicroscopically, numerous mucous droplets of low electron density were prominent in the cytoplasm, and the tumor cells had sparse irregular microvilli on the luminal surface. Mucin histochemistry, including paradoxical concanavalin A staining, revealed that the tumor cells contained neutral and acid mucins, and these were identified as class II and III mucosubstances. No other neoplastic lesion, except recurrent metastatic neck nodes, has been detected 6 years after the first examination, and it seems that the tumor is a rare primary mucinous adenocarcinoma of the submandibular gland. 