Langerhans cell histiocytosis rarely presents as a solitary lesion in the pituitary-hypothalamic region, and is indistinguishable from germinoma, which occurs much more frequently, especially in Japanese. A 14-year-old girl and a 9-year-old girl presented with polydipsia and polyuria as the initial symptoms. Magnetic resonance (MR) imaging demonstrated a round mass at the pituitary stalk appearing as isointense on T₁-weighted imaging and hyperintense on T₂-weighted imaging. Endocrinological examination revealed mild hypopituitarism with central diabetes insipidus. Both patients underwent open craniotomy. Histological examination revealed granulomatous tissue with eosinophil infiltration and frequent Langerhans histiocyte clustering, compatible with the diagnosis of Langerhans cell histiocytosis. Low-dose local irradiation of 20 Gy was administered. First patient was followed up for 8 years, and her hypopituitarism gradually improved to a minimal level with only amenorrhea as the residual symptom. Recent MR imaging showed no residual mass at the region. Second patient was followed up for 15 months, and her diabetes insipidus is stable. MR imaging performed 5 months after the treatment showed marked reduction of the mass. These cases reemphasize the importance of histological diagnosis for lesions with similar neuroimaging appearances. Biopsy and low-dose irradiation are an effective treatment for this rare and essentially benign lesion, as opposed to attempting total removal of the mass.