Early clinical observations and recent systematic studies overwhelmingly document a greater role for psychosocial stressors in association with the first episode of major affective disorder than with subsequent episodes. The author postulates that both sensitization to stressors and episode sensitization occur and become encoded at the level of gene expression. In particular, stressors and the biochemical concomitants of the episodes themselves can induce the protooncogene c- fos and related transcription factors, which then affect the expression of transmitters, receptors, and neuropeptides that alter responsivity in a long-lasting fashion. Thus, both stressors and episodes may leave residual traces and vulnerabilities to further occurrences of affective illness. These data and concepts suggest that the biochemical and anatomical substrates underlying the affective disorders evolve over time as a function of recurrences, as does pharmacological responsivity. This formulation highlights the critical importance of early intervention in the illness in order to prevent malignant transformation to rapid cycling, spontaneous episodes, and refractoriness to drug treatment.