A 45-year-old woman had pyrexia, headaches, collapse and hyponatraemia. Intracerebral abscess, bacterial meningitis and subarachnoid haemorrhage were excluded. She was given intravenous antibiotics and gradually recovered. One month later she was readmitted with diplopia, headache and vomiting. Serum sodium was low (107 mmol/l) and a diagnosis of inappropriate ADH secretion was made. MRI scan showed a suprasellar tumour arising from the posterior pituitary gland. A skin rash gradually faded. Serum cortisol, prolactin, gonadotrophins and thyroid hormone levels were low. A pituitary tumour was removed trans-sphenoidally, she had external pituitary radiotherapy, and replacement hydrocortisone and thyroxine. She was well for 12 months when she developed progressive weakness and numbness of both legs. Examination suggested spinal cord compression at the level of T2 where MRI scanning showed an intradural enhancing mass. This spinal tumour was removed and her neurological symptoms disappeared. Nine months after this she developed facial pain and nasal obstruction. CT scan showed tumour growth into the sphenoid sinus and nasal cavities. A right Cauldwell-Luc operation was done and residual tumour in the nasal passages was treated by fractionated external radiotherapy and Prednisolone. Histological examination of the specimens from pituitary, spinal mass, and nasal sinuses showed Rosai-Dorfman disease, a rare entity characterized by histiocytic proliferation, emperipolesis (lymphophagocytosis) and lymphadenopathy. Aged 48 she developed cranial diabetes insipidus. Although Rosai-Dorfman syndrome is rare, it is being reported with increasing frequency, and should be borne in mind as a possible cause of a pituitary tumour.