Authors report on a rare case of thyroid involvement by histiocytosis X. A 27-yr-old man with diabetes insipidus and lung bullous disease of 2 yr duration was found to have thyroid gland infiltration by differentiated histiocytosis X. This was based on cytological findings consistent with the diagnosis, obtained by fine needle aspiration biopsy. Endocrine studies revealed altered hypothalamic-pituitary function accounted for by a hypothalamic lesion.