The field of biological markers of affective disorders is defined on its conceptual and methodological basis, and the results of some studies in this area conducted at the Mexican Institute of Psychiatry are reported. Urinary MHPG levels greater than 2800 micrograms/24 hs, suggest a poor response to conventional drug treatment. Allnight EEG recordings, showed that sleep architecture of depressed patients is substantially different from that of normal subjects; particularly REM sleep latency, which can reliably discriminate between patients and controls. The dexamethasone suppression test showed a diagnostic confidence of 77% which is similar to that reported from other centers. The author suggests caution on interpreting these results, as further prospective longitudinal studies are needed before firm conclusions can be drawn.