The aim of the present study was to determine the efficacy of any antipsychotic medication for treating people with a dual diagnosis of intellectual disability and schizophrenia. The authors performed an electronic search of Biological Abstracts, the Cochrane Schizophrenia Group's Register of trials, the Cochrane Library, EMBASE, PsycLIT and MEDLINE. Unpublished data were sought from pharmaceutical companies. Both authors independently selected the relevant studies from the reports identified in this way. Only one relevant randomized trial was found by the searches (Foote 1958). This study included four people with a dual diagnosis of schizophrenia and intellectual disability, but results were only available for two subjects. The groups to which the other two people were allocated were unclear. Using the methods described, the reviewers found no trial evidence to guide the use of antipsychotic medication for those with both intellectual disability and schizophrenia. Until the urgent need for randomized controlled trials is met, clinical practice will continue to be guided by evidence from trials involving people with schizophrenia but without intellectual disability.