Although lithium augmentation is the foremost and most well-documented treatment strategy for treatment resistant depression, knowledge of factors related to response remains scanty. Findings with the combined dexamethasone/corticotropin-releasing hormone (DEX/CRH) test are associated with response to treatment with a tricyclic antidepressant. This study investigated the potential predictive value of the DEX/CRH test for lithium augmentation response in major depressive disorder. The DEX/CRH test was conducted prior to lithium augmentation in 30 patients with a major depressive episode who had not responded to an antidepressant monotherapy trial of at least 4 weeks. Response status was assessed weekly using the Hamilton Rating Scale for Depression. For multivariate prediction, a logistic regression analysis was performed. Eleven (37%) patients responded to lithium augmentation within 4 weeks. Responders showed higher ACTH response and lower cortisol response in the DEX/CRH test, but results were not statistically significant. However, non-responders had a statistically significant higher cortisol/ACTH peak ratio (3.43+/-1.75) compared to responders (2.18+/-1.38) (P=0.027). This ratio is an indicator for the sensitivity of the adrenal cortex to ACTH. A cortisol/ACTH peak ratio of 1.8 was identified as the best cutoff point to differentiate responders from non-responders. In conclusion, results suggest a more sensitive adrenal cortex in non-responders to lithium augmentation. The findings would be in line with the assumption of a more chronic course of depression with more pronounced biological alterations in the non-responder group, because chronic depression is known to cause enlargement of the adrenal gland with a subsequent hypersensitivity to ACTH. Results of this study should be confirmed in a larger study group.