SummaryEighty-one psychotic (" endogenous ") depressions were studied to determine the mode of onset of the symptoms, using a graphic method. Four patterns emerged from this investigation: (1) sudden onset (1 1); (2) gradual onset (50) ; (3) neurotic onset (14) ; and (4) fluctuating onset (6).Groups 1, 2, and 3 were examined further to see if other differences could be found. The results seemed to indicate that group 1 (sudden onset) comprised illnesses that were separate and distinct from those of groups 2 and 3, corresponded to the depressed phase of manic-depressive psychosis, and were truly endogenous; while the depressions of groups 2 and 3 were mainly exogenous, and, though probably related to each other, showed different responses to antidepressive drugs, group 2 responding more consistently.A classification of psychotic depressive illnesses, based on the evidence presented, is set out.