Abstract: This invention relates to the field of treatment, prevention, and identification of persons at high risk of developing major depressive disorder based upon elevated concentration of serotonin 1A receptors (i.e., elevated serotonin 1A receptor binding potential). In particular, the current invention identifies offspring of parents with major depressive disorder who are at significantly elevated risk of developing major depressive disorder before any symptoms have occurred and treats these patients prior to any symptoms and more importantly before developing major depressive disorder. The invention also relates to the identification of persons who have discontinued treatment for major depressive disorder based upon elevated concentration of serotonin 1A receptors (i.e., elevated serotonin 1A receptor binding potential).