Reactive psychosis is a diagnostic concept widely used in Scandinavia but it is still a subject of discussion and controversy internationally. Inside Scandinavia the diagnosis is used according to varying criteria, which is one of the reasons for high diagnostic instability and a threat to the validity and reliability of the concept. This article presents an attempt to operationalize reactive delusional psychosis, to test its clinical utility in a pilot study, and to test the reliability of the individual items. More clinical studies based on operationalization, demarcation of the concept, and interrater reliability are needed to convince psychiatrists outside Scandinavia of the existence of reactive psychosis as a useful nosological concept in patients with good prognosis and who mainly need psychotherapy.