The objective was to review the literature on the biological correlates of major depressive disorder (MDD) in children and adolescents. A computerized search for articles published during the last 20 years was done and selected studies presented. To date, examination of growth hormone (GH), prolactin, and cortisol levels after pharmacological stimulation have shown abnormalities in the secretion of these hormones (e.g. blunted GH secretion after the administration of growth hormone releasing hormone). Identical results have been found in never-depressed children at high risk to develop MDD due to high family loading for MDD suggesting that alteration in certain hormonal systems may be trait markers for MDD. Other biological studies (e.g. the hypothalamic-pituitary axis, sleep electroencephalogram) have yielded more inconsistent results with subjects with melancholic symptoms, severe depressions, and older age showing some abnormalities similar to the ones reported in adults with MDD. Factors such age, sex, maturation, psychiatric family history and exposure to stress need to be considered since they also affect the same biological systems associated with the aetiology of MDD. Considerable biological research has been done in youth with MDD. Further research is needed to investigate whether these markers predict the development of new episodes of MDD, recurrences, and treatment response. Also, these and other studies using more sophisticated methods (e.g. functional MRI) aimed at elucidating the interrelationship between biological and other risk factors are needed.