• The relationship between DSM-III diagnosis and defense style was studied in a sample of 74 psychiatric patients. The subjects completed a self-administered questionnaire composed of 81 statements about behaviors, thoughts, and feelings, which were designed to reflect various unconscious defense mechanisms. Factor analysis yielded four defense styles ranging on a continuum from primitive to mature. Patients' defense styles were compared with their diagnoses on axes I, II, and IV of DSM-III. Significant relationships were found between defense style and only one diagnosis, major affective disorder. The fact that DSM-III diagnosis could not predict defense style suggests that diagnosis and defense style are two independent dimensions. If this can be confirmed by further, more adequate studies, a sixth axis reflecting psychodynamic formulation would provide additional information necessary for therapeutic planning.