Retrospective comparisons between primary unipolar depression and depression secondary to anxiety in 65 inpatients revealed a number of differences. Secondary depression was associated with a significantly higher incidence of neurotic traits in childhood, chronic unhappiness, and unsupportive family. Tricyclic antidepressants and ECT were both more effective in primary depression, and some secondary depressives became worse on ECt. When primary depression was sub-divided into familial, nonfamilial and spectrum types, the greatest differences were noted between familial and secondary depressions. In the former group a more stable life style was noted. Secondary and spectrum types differed on only two variables and several similarities were noted. Platelet monoamine oxidase activity was significantly higher in secondary depression.