Background: The proposed revision of the ICD-10 category of ‘acute and transient psychotic disorders' (ATPDs), subsuming polymorphic, schizophrenic or predominantly delusional syndromes, would restrict their classification to acute polymorphic psychotic disorder, reminiscent of the clinical concepts of bouffée délirante and cycloid psychosis. Sampling and Methods: We selected all subjects aged 15-64 years (n = 5,426) who were listed in the Danish Psychiatric Central Register with a first-admission diagnosis of ATPDs in 1995-2008 and estimated incidence rates, course and outcome up to 2010. Results: Although about half of ATPD patients tended to experience transition to another category over a mean follow-up period of 9.3 years, acute polymorphic psychotic disorder fared better in terms of cases with a single episode of psychosis and temporal stability than the subtypes featuring schizophrenic or predominantly delusional symptoms. Acute polymorphic psychotic disorder was more common in females, while cases with acute schizophrenic features predominated in younger males and evolved more often into schizophrenia and related disorders. Conclusions: These findings suggest that acute polymorphic psychotic disorder exhibits distinctive features and challenge the current approach to the classification of ATPDs.