Light and electron microscopic study of the thyroid gland and an enlarged cervical lymph node in a 75-year-old woman with Hashimoto's thyroiditis disclosed immunoblastic proliferation in the lymph node, marked by collections of striking round cells positive to periodic acid-Schiff (PAS) staining, immunoblasts and plasmacytoid elements in a vascular, fibrous framework. The PAS-positive cells ("macaroni cells") were distended with whorls of angulated tubular material resembling endoplasmic reticulum. Parafollicular-cell hyperplasia and an atypical plasmacytoid focus were noted in the thyroid. Hashimoto's disease is known to be associated with malignant lymphoma, as are autoimmune and malignant diseases with immunoblastic lymphadenopathy. This is the first report of the association of Hashimoto's disease and immunoblastic lymphadenopathy. The atypical plasma cells have not previously been described.