• The genetic relationship between paranoid psychosis (delusional disorder) and schizophrenia spectrum disorders (schizophrenia and schizotypal personality disorder) is examined by a blind independent diagnostic evaluation of the interviews of relatives from the greater Copenhagen sample of the Danish Adoption Study of Schizophrenia. While cases of schizophrenia spectrum disorders are strongly concentrated in the biologic relatives of the schizophrenic adoptees, this pattern is not found for delusional disorder. These results suggest that from a genetic perspective, delusional disorder is not part of the schizophrenia spectrum.