The aim of this work is to investigate differences between acute and transient psychotic disorders (ATPD; F23 of ICD-10) and bipolar schizoaffective disorders (BSAD). In a controlled prospective and longitudinal study, we compared all inpatients with ATPD treated at Halle university hospital during a 5-year period with matched controls with BSAD. Sociobiographical data were collected using a semi-structured interview. Follow-up investigations were performed at a mean of 2.2-3.3 years after the index episode or 8.2-16.1 years after the first episode by means of standardized instruments. ATPD differs significantly from BSAD on various relevant levels, such as gender (more female), age at onset (older), development of the full symptomatology (more rapid), duration of the symptomatology (shorter), acuteness of onset (more acute), preceding stressful life-events (more frequent) and long-term prognosis (better). It is concluded that ATPD and BSAD are different nosological entities.