Recent studies have suggested that expanded CAG/CTG repeats contribute to the genetic aetiology of schizophrenia and bipolar disorder. However, the nature of this contribution is uncertain and difficult to predict from other known trinucleotide repeat diseases that display much simpler patterns of inheritance. We have sought to replicate and extend earlier findings using Repeat Expansion Detection in an enlarged sample of 152 patients with schizophrenia, 143 patients with bipolar disorder, and 160 controls. We have also examined DNA from the parents of 62 probands with schizophrenia or bipolar disorder. Our results confirm our earlier, preliminary findings of an association between expanded trinucleotide repeats and both schizophrenia and bipolar disorder. However, our data do not support the hypothesis that trinucleotide repeat expansion can alone explain the complex patterns of inheritance of the functional psychoses neither can this mechanism fully explain apparent anticipation.