The present study investigated the occurrence and the clinical correlates of psychiatric co-morbidity in a sample of 64 patients with delusional disorder (DD). Subjects were evaluated with a semi-structured interview for the collection of demographic and clinical features of the disorder; co-morbid axis 1 disorders were determined according to the clinical interview using DSM-IV by Othmer and Othmer. Delusional disorder subjects with and without co-morbid diagnoses were compared to investigate whether the presence of another psychiatric disorder influenced the clinical features of the illness.Seventy-two percent of the subjects had at least one additional lifetime psychiatric diagnosis. High lifetime co-morbidity was found with affective disorders, whose onset generally had been subsequent to the onset of DD. Patients with at least one co-morbid disorder (N = 46) had an earlier age at onset, presented for the first psychiatric consultation at an earlier age, and were younger at index evaluation for this study with respect to patients without co-morbidity (N = 18). Types of DD differed significantly according to the presence/absence of lifetime co-morbid disorders: DD patients with co-morbidity were in most cases persecutory type (54.4%) while DD patients without co-morbidity were mixed type (66.7%). Our data indicate that there is a considerable proportion of patients whose DDr is strictly connected with other co-occurring psychiatric disorders (mainly affective disorders), which exert an influence on the phenomenology of the illness.