Harlequin syndrome is considered to be an idiopathic, benign condition characterised by sudden onset hemifacial sweating and flushing induced by exercise and heat. In light of this we describe a 17-year-old man who presented due to social embarrassment after a 3-year history of troublesome sweating which affected the right side of his face, neck and the upper shoulders. This was associated with, and precipitated by exercise, emotional stress and exposure to heat. Examination was unremarkable apart from excessive sweating on the right side of his face, neck and shoulders with a strict demarcation at the midline. A magnetic resonance scan (MR) scan of the cervical and thoracic spine demonstrated a nonenhancing right-sided intrinsic cord lesion extending from C5 to T1 consistent with a low grade infiltrative tumour. This was associated with a syrinx. Blood investigations were within normal limits. Harlequin syndrome is a rare disorder of localised failure of the upper thoracic sympathetic chain resulting from a contra-lateral sympathetic deficit often secondary to a peripheral lesion which affects the sympathetic outflow. Harlequin syndrome has been reported in association with recognised neurological disorders and also with external mediastinal compression however central structural lesions are rarely reported to be responsible for the syndrome.