An 80-yr-old female with a grade I diffuse goiter was diagnosed on fine-needle aspiration cytology as having Hashimoto's thyroiditis. Two years later, she developed a hard nodular growth in the right lobe of the thyroid, which was subjected to fine-needle aspiration and diagnosed as a non-Hodgkin's lymphoma. Immunocytology demonstrated the monoclonal B cell nature of the lesion. The patient underwent total thyroidectomy and is alive and free of the disease 27 mo after surgery.