The finding of contrasting results regarding an association between schizophrenia and the MscI polymorphism site in the dopamine D3 receptor gene prompted us to study the distribution of this polymorphism in an Italian sample of 52 patients suffering from delusional disorder, 85 schizophrenic patients and 78 control subjects. No significant differences in genotype, allele and homozygosity frequencies between schizophrenics and controls were found, while a significant, albeit modest, association was discovered between delusional disorder and both the 1 allele and the 1-1 genotype. This suggests a contribution of this gene to the liability to develop delusional disorder.