Two hundred and forty-five cases of malignant lymphoma and other lymphoproliferative disorders involving the thyroid gland have been studied. The patients, predominantly elderly women, usually presented with a rapidly enlarging mass in the neck, often with symptoms of tracheal or laryngeal compression. Radioisotopic scanning of the thyroid usually demonstrated a "cold" or "cool" process, and tracheal deviation (as seen by chest roentgenogram) was common. Lymphoma and related processes were nearly always associated with an underlying lymphocytic thyroiditis. The mean length of follow-up was 45 months, and follow-up was obtained in 96% of the cases. An unfavorable prognosis was related to extension of tumor cells outside the capsule of the gland, necrosis of the tumor, blood vessel wall infiltration, diffuse architectural pattern, and cervical lymph node involvement, with or without disease outside the neck. Tumors composed entirely of plasma cells demonstrated little if any tendency toward systemic extension, and those lymphomas with some plasmacytoid differentiation also displayed a more favorable prognosis.