A 30-year-old female presented to her general practitioner with a three-month history of fatigue, visual disturbance, polydipsia and dizziness. She was treated for iron deficiency anaemia, but re-presented two-months later with new-onset headache and worsening visual disturbance. Previously, she had childhood leukaemia, treated in Brazil with no cranial irradiation. On examination there was left eye loss of light/dark perception and right temporal vision loss. She was referred to our centre for further investigation. Blood tests revealed a sodium 158 mmol/l, urea 3.5 mmol/l, serum osmolality (calculated) 325 mOsm/kg and urine specific gravity 1.010. The patient was thirsty and drinking around 6 l/day. MRI head confirmed the presence of a large sellar mass with suprasellar extension, compression of the optic chiasm and hypothalamus and a similar second lesion. Pituitary function tests demonstrated raised serum prolactin (1729 mU/l, macroprolactin negative), morning cortisol 301 nmol/l and secondary hypothyroidism (free T4 7.7 pmol/l, TSH 0.33 mU/l). The patient started prednisolone 5 mg daily and subsequently 50 (g levothyroxine; she also started nasal DDAVP two sprays daily. Her serum hCG and AFP were normal, but an elevated CSF hCG (16 IU/l, normal <2) indicated a diagnosis of intracranial germ cell tumour. She was started on emergency etoposide/cisplatin (EP) chemotherapy followed by EP/OMB (vincristine, methotrexate, bleomycin) chemotherapy and intrathecal methotrexate. Three days later her visual perimetry had normalised and repeat imaging demonstrated a marked decrease in the suprasellar mass. Intracranial germ cell tumours can present with cranial diabetes insipidus (DI) as a result of compression of or invasion into the pituitary stalk. This tumour is an important differential in a young person and the case illustrates the need to assess CSF tumour markers when the serum values of hCG and/or AFP are normal.