Few studies have investigated the long-term course of patients with acute polymorphic psychotic disorder, despite the clinical relevance. The present study focused on the frequency of acute polymorphic psychotic disorder without symptoms of schizophrenia (F23.0) and the prognosis of patients with the disorder over a long period. A total of 388 inpatients with nonaffective psychosis were examined and cases with F23.0 were retrospectively investigated regarding clinical course over 12 years with reference to clinical charts. A total of 16 cases were diagnosed with F23.0 according to first episode. After 12 years, five cases were rediagnosed with schizophrenia, and one case with borderline-type emotionally unstable personality disorder, although the diagnosis of F23.0 was not changed in 10 older patients. Of the four cases with >2 repeated episodes of F23.0, the same clinical picture as in the first episode was observed in three, while one case showed continuous residual symptoms. F23.0 does not represent a homogeneous clinical entity. In some cases, diagnosis changes to schizophrenia, but others display repeated episodes of F23.0.