Abstract
The atypical features subtype of major depressive disorder was introduced into DSM-IV largely on the basis of the preferential response shown by a subset of depressed patients to monoamine oxidase inhibitors. In the present report, studies evaluating the biological nature of atypical depression are reviewed to determine whether they support the existence of this subtype. Four lines of research are considered: studies involving hypothalamic-pituitary-adrenal axis activity, cerebral laterality, neurochemical profiles, and sleep parameters. Taken together, the findings of these investigations do support the existence of the atypical features subtype. Furthermore, many of the features known to be associated with atypical depression, such as higher prevalence in women, higher prevalence of comorbid anxiety disorders, and younger age of onset, may be accounted for by the biological underpinnings of the disorder.