Lytic bone lesions (sometimes diagnosed incidentally on radiography) are clinical hallmark; also marked by diabetes insipidus, chronic otitis, fever, and weight loss. Definitive diagnosis requires presence of Birbeck granule-positive cells in lesions seen on electron microscopy. Classically, when bone lesions only are present, disease is termed eosinophilic granuloma. When bone lesions, diabetes insipidus, and exophthalamos are present, disease is termed Hand-Schuller-Christian disease. The disseminated form is termed Letterer-Siwe disease.