An abnormal dexamethasone suppression test (DST) identifies endogenous depressive with 95% confidence. The DST normalizes with successful treatment. Serial DSTs throughout treatment could provide useful information on (i) whether they biologically monitor clinical response, (ii) when DST normalization does occur, and (iii) whether early normalization predicts clinical outcome. We studied six inpatients who had abnormal DSTs on admission and serial DSTs during and after a course of ECT. All were diagnosed as unipolar major depressive disorder, endogenous (RDC). Clinical improvement was monitored with weekly Hamilton Rating Scales. Five of six abnormal DST patients converted to a normal test and showed significant clinical improvement. On patient failed to normalize and also had a poor outcome. All DST normalizations occurred by the fourth to sixth ECT. Additional ECTs resulted in further reduction in DST plasma cortisol values. All normalizers experienced progressive clinical improvement, and the DST successfully monitored these changes. DST conversion occurred early in treatment. Early DST normalization may have predictive value for good treatment response.