A 51-year-old man presenting with paroxysmal attacks of unilateral sweating for 1 year was studied. Clinical and laboratory findings made the diagnosis of Shapiro or Harlequin syndrome unlikely. The onset of the sweating "crisis" was immediately followed by a progressive decrease in rectal and skin temperature, lasting for about 2 hours. This indicated that hypothermia was related to an abnormal downward shift of the thermoregulatory set point and a delayed effect of mechanisms blocking heat dissipation.