The ICD-11 International Classification of Diseases and Related Health Problems will move toward a narrower concept of “acute and transient psychotic disorders” (ATPD) characterized by the remnant “polymorphic psychotic disorder” (APPD) of the current ICD-10 category, also including schizophrenic and predominantly delusional subtypes. To assess the validity of APPD, relevant articles published between January 1993 and September 2017 were found through searches in PubMed and Web of Science. APPD is a rare mental disorder and affects significantly more women than men in early-middle adulthood. Its diagnostic reliability is relatively low, and its consistency reaches just 53.8% on average over 8.8 years, but is significantly greater than either of ATPD subtypes, which are more likely to progress to schizophrenia and related disorders. Although APPD has distinctive features and higher predictive power, its rarity and the fleeting and polymorphic nature of its symptoms could reduce its usefulness in clinical practice and discourage research.