Expansion of triplet repeats has been seen to underlie several disorders that manifest anticipation. Clinical evidence suggests that anticipation occurs in the major psychoses. We studied the distribution of repeat sizes at the Machado-Joseph disease (MJD) locus in a group of patients with the major psychoses. We did not find any large expansions, though 2 patients had alleles that were two repeats larger than in our controls. The difference in allele sizes was larger in the patient sample as compared to the controls. The effect of such large differences might be of functional significance.