We present the case of a 19-year old male who presented with a generalized tonic clonic seizure associated with visual loss. Examination revealed severe visual field defects and arrested pubertal development. Laboratory evaluation revealed a very elevated prolactin of 298 410 mU/l, hypogonadotrophic hypogonadism, secondary adrenal insufficiency, and secondary hypothyroidism. He was also found to have thrombocytosis due to JAK 2 essential thrombocythaemia. Pituitary MRI revealed a large pituitary macroadenoma (58×40×28 mm) exerting significant pressure on the optic chiasm, associated with acute hydrocephalus. X-rays of the hands and wrist revealed delayed bone age of 16 years. The patient was diagnosed with a giant prolactinoma. Treatment was initiated with cabergoline 0.5 mg daily, hydrocortisone 5 mg BD, thyroxine 75 mcg daily and testosterone 100 mg IM injection every 4 weeks. Despite an impressive and rapid reduction in tumour size and prolactin levels, the visual defects and hypogonadal axis have not recovered after 24 months of therapy. Giant prolactinomas represent 0.5% of all pituitary adenomas (1). They are characterised by their size (>40 mm) and extremely high prolactin levels. The most common presentations include visual field defect, headache and sexual dysfunction, often accompanied by hypopituitarism (1). The goals of treatment are to relieve acute compressive symptoms, reduce tumour mass, normalise prolactin levels, and preserving pituitary function (2). Dopamine agonists are first line therapy for giant prolactinomas and can rapidly decrease tumour size and prolactin levels. Give his age, he will undergo testing for AIP and MEN mutations. Prolactin belongs to family of cytokines using the JAK-STAT signal transduction pathway, which regulates cellular proliferation and apopotosis (3). Constitutional activation of JAK2/STAT 5 pathway has been implicated in variety of tumours, however there has no previous account of lactotroph proliferation. We report the first case of JAK2 mutation in association with giant prolactinoma.