PURPOSE: To examine adult epilepsy outpatients for the existence of the interictal dysphoric disorder (IDD) using the interictal dysphoric disorder inventory (IDDI), the overlap between IDD, depression, and anxiety, and the reproducibility of IDDI. METHODS: Epilepsy outpatients were assessed with the Danish IDDI and self-report inventories for depression and anxiety. Patients with abnormal scores were further assessed with the Mini International Neuropsychiatric Interview (MINI). Patients with IDD were asked to repeat IDDI for evaluating the reproducibility. Quality of life, well-being and adverse effects to antiepileptic drugs were determined. RESULTS: We included 169 patients, and 32 (19%) were diagnosed with IDD. Thirty patients were further assessed with MINI, and 17 (57%) were diagnosed with additional psychiatric disorders, mainly depression, dysthymia, and anxiety. Patients with IDD and additional psychiatric comorbidity had significantly higher seizure frequency, higher level of side effects to the antiepileptic treatment, and lower quality of life, both when compared to patients with normal screening and patients with IDD as the only comorbidity. The reproducibility of the Danish IDDI was only 50%. CONCLUSION: With a prevalence of 19%, IDD appeared to be the commonest neuropsychiatric syndrome. The majority of the patients with IDD also had depressive and/or anxiety disorders. Quality of life, seizure control, and side effects to antiepileptic drugs were affected much more by depression or anxiety, than by IDD. The Danish version of IDDI has a poor reproducibility. The existence of IDD as a diagnostic entity is doubtful.