A mixed-breed dog, evaluated because of a chronic history of diarrhea and intestinal bleeding, was found to have a multilobular mass involving the intestine. Surgical resection of the mass necessitated an ileocolic anastomosis. On the basis of histologic and electron microscopic appearance and staining characteristics, a tentative diagnosis of intestinal carcinoid was made. Recovery was uncomplicated; the owners declined further diagnostic tests or treatment. Eight weeks later, the dog was reexamined because of signs consistent with hyperviscosity syndrome and hyperproteinemia. At the owner's request, the dog was euthanatized. At necropsy, extensive metastases to liver and lymph nodes, but not to bone, were seen. Reevaluation of the intestinal mass supported classifying the tumor as an IgG-secreting extramedullary plasmacytoma. This case underscores the need for additional histologic techniques, especially when confronted with an unusual manifestation of hyperproteinemia.