Harlequin syndrome (HS) is a rare disorder, characterized by hemifacial flushing and sweating. This condition is induced by exercise, heat and emotional factors. There are sudomotor and vasomotor deficits in the cervical sympathetic chain. The syndrome is usually benign and does not require treatment. Most of the HS cases are primary in nature and no underlying cause can be identified, but may be associated with a secondary cause such as organic lesion. Here, we diagnosed idiopathic HS in a 20-year-old female who presented with increased facial flushing and sweating in the left half of the face which were mainly induced by exercise and emotional factors and it was reported together with literature.