An account of the response to biological treatment of 20 patients with the syndrome of doubles (syndrome of Capgras, syndrome of Frégoli, syndrome of Intermetamorphosis, syndrome of subjective doubles) is presented. Our observations indicate that: a) the syndrome of doubles may respond to various biological treatment methods; b) in the setting of depression it responds to tricyclic antidepressants; c) in the setting of schizophrenia or organic psychosis it usually responds to neurolytics; d) in a schizophrenic setting it has more chances of responding to trifluoperazine either given alone or in association with other psychopharmaca, and, e) in certain cases combination of antipsychotic treatment with treatment of co-existing organic dysfunction appears to be important.