The effect of thyrotoxicosis upon the recurrence of manic-depressive psychoses has been studied by the use of a routine follow-up system. In this system 267 patients had three or more affective illnesses. Five of these patients had eight well-documented episodes of thyrotoxicosis. Only three of these eight episodes coincided with an affective illness, and in each case an alternative explanation for the association was available. These findings suggest that thyrotoxicosis has little effect upon the occurrence of a manic-depressive episode.