We report a carcinoma that is, to the best of our knowledge, the first case of a mixed mucus-secreting and oncocytic carcinoma of the thyroid. We also describe the histochemical, immunohistochemical, and ultrastructural features of this tumor. A 59-year-old man complaining of severe bone pain and weight loss underwent clinical and radiologic investigations. The studies revealed a nodule in the left thyroid lobe that was "cold" by (131)I scintiscan and multiple lytic lesions of the skeleton that showed increased uptake by (99m)Tc-Sestamibi scintiscan. Left hemithyroidectomy was performed and the surgical specimen contained a well-circumscribed nodule of 3 cm in the greatest diameter. Light microscopy showed an oncocytic carcinoma with an area of glandular and papillary proliferation of mucin-producing cells. A double histochemical approach (Alcian blue-periodic acid-Schiff and Alcian blue-high-iron diamine) combined with ultrastructural investigation confirmed the presence of true mucus, ruling out the presence of breakdown products of thyroglobulin. Ultrastructural and immunohistochemical studies, together with clinical findings, excluded a possible metastatic origin of the mucin-producing component.