An apparent excess of sex chromosome aneuploidies (XXY, XXX, and possibly XYY) has been reported in populations of patients with schizophrenia by a number of authors. These reports have received little attention because transmission of psychosis is regarded as autosomal and not sex linked, and the detection of extra X chromosomes by Barr body estimation alone is not a reliable procedure. In this article, we review studies in which either complete karyotypes were determined for the whole sample or in which the presence of a Barr body in an individual was checked by full cytogenetic analysis. We also add two studies (of the former type) of our own—on a Swedish hospital cohort and a United States multiplex-schizophrenia family sample. These data, taken together, suggest that the sex chromosome aneuploidies, XXX and XXY, are increased in populations of patients with schizophrenia, whereas too few subjects have been surveyed to determine whether an association also exists with XYY. Nevertheless, we conclude that this is consistent with a gene on the sex chromosomes having influence on the development of schizophrenia. A sex chromosome locus is compatible with an autosomal pattern of transmission if the gene is either pseudoautosomal (i.e., within the exchange region) or X-Y homologous (i.e., present in similar form in the nonrecombining regions of both X and Y chromosomes).