Histiocytosis X rarely involves the thyroid gland and occurs as one of the unusual manifestations of systemic diseases. Diagnosis is based on the identification of the Langerhans' cells in an appropriate background. We report a case of histiocytosis X confined to the thyroid gland that represents the primary manifestation of the disease. A careful histological, immunohistochemical and ultrastructural study has been performed and problems of differential diagnosis are discussed.