The prevalence of an abnormal response to the dexamethasone suppression test (DST) was examined in 119 in-patients suffering from a major depressive disorder and in 79 normal controls. Only 11 per cent of controls showed an abnormal DST as against 70 per cent of depressed patients. The specificity of the DST was examined by testing patients with other psychiatric disorders. Abnormal responses were found in one-fifth of a sample of schizophrenics, over one-quarter of abstinent alcoholics, two-fifths of neurotics (including neurotic depressives) and almost half of senile dements. Abnormal DST was also found in 33 per cent of patients receiving prophylactic lithium for recurrent affective disorders.