A 31-year-old man was admitted to our hospital because of a sudden onset of thirst, polyposia, and polyuria. Five years previously he had been admitted to our hospital because of a dry cough. On the first admission, the chest X-ray film had shown reticular shadows and bullous changes in both upper lung fields. Histological examination of a transbronchial lung biopsy specimen had revealed that the nodular lesion in the interstitium of the alveolar lesion consisted of an aggregate of many Langerhans cells with pale cytoplasm and partly convoluted nuclei. In addition, immunoperoxidase stain for S-100 protein had been strongly positive in numerous Langerhans cells in a bone biopsy specimen from a left mandibullar lesion, which is the same histological appearance as the lung lesion. A diagnosis of pulmonary eosinophilic granuloma had been made. The course after discharge was not progressive without treatment for 5 years, but the patient suddenly began to have thirst, polyposia, and polyuria. Dehydration, vasopressin tests, and the findings of MRI indicated diabetes insipidus due to a pathological change in the pituitary gland. Although diabetes insipidus is known to be a common complication of pulmonary eosinophilic granuloma, only 9 cases have been reported in Japan.