Response to pharmacologic treatments may identify groups of disorders with a common pathophysiology. The authors applied a treatment-response model, based on four classes of antidepressants (tricyclic types, monoamine oxidase inhibitors, serotonin uptake inhibitors, and atypical agents), to the medical literature. The model identified eight disorders that may share a pathophysiologic abnormality: major depression, bulimia, panic disorder, obsessive-compulsive disorder, attention deficit disorder with hyperactivity, cataplexy, migraine, and irritable bowel syndrome. Phenomenologic and family studies support this grouping. If the model is validated, this family of disorders, which the authors term "affective spectrum disorder," would represent one of the most prevalent diseases in the population.