Central diabetes insipidus with an onset in adulthood is very rare. Unlike in children, central diabetes insipidus in adults is more frequently caused by inflammatory processes and neoplastic infiltrations that do not originate from the neuronal tissue than primary neuronal tissue tumours. Rare histiocytic neoplasias (Langerhans cell histiocytosis, xanthogranulomatosis and Erdheim-Chester disease) have a specific affinity to hypothalamus and the pituitary stalk not only in paediatric patients but also when occurring in adults. We describe 3 cases of central diabetes insipidus with an onset in adulthood. Diabetes insipidus was the first sign of Langerhans cell histiocytosis in 2 patients, and it was the first sign of Erdheim-Chester disease in one patient. MR imaging showed pathological infiltration and dilated pituitary stalks in all 3 patients. PET-CT proved useful in differential diagnosis, showing further extracranial pathological changes either on the basis of significant glucose accumulation or on the basis of CT imaging. The Langerhans cell histiocytosis in the first patient has also manifested itself as an infiltration of the perianal area with intensive accumulation of fluorodeoxyglucose (FDG) - SUV 8.6 and gingival inflammation indistinguishable from parodontosis. Histology of the perianal infiltrate confirmed Langerhans cell histiocytosis. Infiltration of the pituitary stalk disappeared from the MR image after 4 cycles of 2-chlordeoxyadenosin (5 mg/m2 5 consecutive days). The PET-CT of the 2nd patient showed only borderline accumulation of FDG in the ENT area, while simultaneously performed CT imaging showed cystic restructuring of the pulmonary parenchyma and nodulations consistent with pulmonary Langerhans cell histiocytosis. Bronchoalveolar lavage identified higher number of CD1 and S100 positive elements, consistent, once again, with pulmonary LCH also affecting pituitary stalk and ear canal. The PET-CT of the third patient showed increased activity in the long bones and ilium near the sacroiliac joint. Biopsy of the focus in the ilium confirmed foam histiocyte infiltration immunochemically corresponding to Erdheim-Chester disease. Additional imaging assessments revealed the presence of further signs of the disease. Pituitary infiltrate biopsy in this patient did not elucidate the diagnosis but resulted in complete panhypopituarism. Central diabetes insipidus in adulthood might be the first sign of so far undiagnosed extracranial disease, in our case of histiocytic neoplasias, and PET-CT has an excellent potential to detect extracranial symptoms of these conditions. Therefore, the high-risk pituitary stalk infiltrate biopsy should always be preceded by comprehensive examination aimed at identification of extracranial manifestations of the pituitary gland diseases.