Thyroid function abnormalities are common in persons with rapid-cycling bipolar affective illness. Some women who cycle rapidly respond to thyroxine, but there is scant evidence that the people who have thyroid abnormalities are the ones who respond to thyroid treatment. The authors report successful thyroxine treatment of a man whose rapid cycling began and resolved after changes in his thyroid status. The period of his affective cycle was altered by both lithium and thyroxine. The authors suggest that CNS thyroid status may determine the clinical course of some persons with a fixed cycle of affective episodes.