The group of paranoid or delusional disorders, although not nearly as common as the mood and schizophrenic disorders, may be much more frequent than has usually been thought. DSM-IIIR has made a decisive step in recognizably defining at least one group of them. Interestingly, this change partly came about because the advent of an effective treatment helped to define that group more clearly. Nevertheless, DSM-IIIR's classification is too restrictive, and it was wrong to exclude the diagnosis of paraphrenia. Cases fitting this description will have to be consigned to the category of Psychotic Disorder NOS, which will inevitably be a grab-bag of mixed diagnoses. Also, DSM-IIIR does not emphasize the link between the delusional disorders and paranoid schizophrenia, and the somewhat less well defined overlap with affective disorders, both of which give rise to much diagnostic confusion and inappropriate treatment. Precise history taking and mental status examination and, above all, an up-to-date knowledge of their existence are essential to the recognition and appropriate treatment of the delusional disorders.