The demonstration that an endocrine disorder can precipitate a specific mental illness would be an important step towards the development of much needed animal models of affective disorder. To this end we have sought and found evidence for a causal connection between Cushings syndrome and the apprearance of a depressive illness (Kelly et al, 1980 and 1983). Whether or not hyperthyroidism can cause a manic depressive illness is at present uncertain. Reported values for the incidence of depression in hyperthyroidism vary from 1–20 per cent (Bursten, 1961; Clower et al, 1969; Dunlap and Moersch, 1935; Johnson, 1928; Katzenelbogen and Luton, 1935; Kleinschmidt et al, 1956; Lidz and Whitehorn, 1949) and these are similar to estimates of the incidence of depression in a general population (Watts et al, 1964; Shepherd et al, 1966; Crombie, 1974; Brown and Harris, 1978). Similarly estimates of the incidence of hyperthyroidism in depressed patients are around 1 per cent (Bursten, 1961; Bluestone, 1957; Martin, 1963) which is also similar to the incidence of hyperthyroidism in the general population (Tunbridge et al, 1975). Finally in a population of patients with recurrent manic depressive illness, the eight observed episodes of hyperthyroidism striking during normal health were not apparently followed by any mental disturbance (Checkley, 1978). We now report upon a patient in whom two episodes of mania coincided with two episodes of hyperthyroidism and discuss the possibility that the two illnesses were related to each other.