We report 2 cases of Langerhans cell histiocytosis (LCH) presenting as a thyroid mass. The first case is a 45-year-old woman with a 13-year history of diabetes insipidus who presented with an enlarging thyroid mass with substernal extension. The second case is a 29-year-old man who presented with an enlarging thyroid mass and skin lesions. Histologic evaluation of the thyroid gland in both cases revealed extensive involvement by LCH, confirmed by immunohistochemical analysis showing Langerhans cells that were positive for CD1a and S-100 protein. Langerhans cell histiocytosis can rarely involve the thyroid gland in adults, and we have identified 30 cases reported in literature. Most patients had evidence of LCH involving other anatomic sites, as was true in these 2 cases, and the diagnosis was initially established by examination of other sites in a subset of patients. Affected patients frequently have diabetes insipidus, as was true in case 1. Thyroid gland involvement as the initial presentation of LCH is a rare phenomenon that can result in misdiagnosis.