Background. Several case reports and reviews have suggested an increased incidence of schizophrenia or bipolar disorder among persons with sex chromosome aneuploidies, but this observation may have been caused by biased sampling.

Methods. The 1122 individuals with sex chromosome aneuploidies registered in the Danish Cytogenetic Central Register were screened in the Danish Psychiatric Central Register for admissions with schizophrenia or bipolar affective disorder. Both registers are population based and have existed since 1968 and 1969 respectively. Relative risks were calculated for schizophrenia, bipolar affective disorder and either schizophrenia or bipolar disorder combined as one phenotype. Since hospitalization for a psychiatric disorder increases the probability that a cytogenetic examination is performed, the relative risks could be inflated, and they were therefore adjusted accordingly.

Results. Aneuploidies of the X or Y chromosomes were not associated with an increased risk of schizophrenia or bipolar disorder. The occurrence of the combined phenotype including both schizophrenia and bipolar disorder was significantly reduced among persons with Turner's syndrome and significantly increased among individuals with the 47, XYY karyotype.

Conclusions. This population-based study did not find evidence supporting the presence of risk alleles for schizophrenia or bipolar disorder on the X chromosome or the pseudoautosomal region on the Y chromosome. The findings of an increased risk for the combined phenotype to XYY males and the reduced risk for persons with Turner's syndrome are interesting but difficult to explain with present neurobiological knowledge and inconsistent with the other findings of the study.