The term 'delusional misidentification syndromes' (DMS) encompasses disparate clinical phenomena. This lack of agreement has resulted in a proliferation of definitions and classifications and impeded progress in aetiological research. It is suggested here that one reason for such confusion lies in current overemphasis on the content of such phenomena and a consequent neglect of their form. Research directed at clarifying the protean forms in which misidentifications manifest themselves should help to delineate more stable clinical phenomena and identify causal mechanisms.