To examine temporal patterns and predictors for diagnostic conversion from unipolar depression (UD) to bipolar disorder (BD), schizophrenia, and schizoaffective disorder (SAD).A prospective nationwide register-based cohort (n=43,495) of all first psychiatric hospitalizations due to UD during 1996-2011 was followed up to 15 years. We used cumulative incidence function analyses and the Fine-Gray subdistribution model to define the cumulative incidence ofthe conversions and subdistribution hazard ratios (SHRs) for predictors.The overall 15-year cumulative incidence of conversion was 11.1% (95%CI 10.7-11.6): 7.4% (95%CI 7.0-7.8) for BD, 2.5% (95% CI 2.3-2.7) for schizophrenia, and 1.3% (95% CI 1.1-1.4) forSAD. The highest crude incidence rate emerged during the first year. Psychotic depression predicted higher conversion risk to BD (SHR=2.0, 95% CI 1.5-2.7), schizophrenia (SHR=5.3, 95% CI 3.3-8.7), and SAD(SHR=10.6, 95% CI 4.0-28.4) than mild depression. Female sex, greater overall disturbance, and comorbid personality disorderpredicted conversion to BD, whereas young age and male sex to psychotic disorders.Among patients with first hospitalization due to UD, approximately one in nine converts to another major psychiatric disorder during 15 years, withthe highest risk occurringwithin the first year. Patients with psychotic depression are particularly vulnerable for conversion to othermajor psychiatric disorders.Conversion to psychotic disorders occurs earlier than to BD. Males are at higher risk for progression to psychotic disorders, whereas females, patients with recurrent depressive episodes, severe disturbance of overall functioning and personality disorder are at higher risk for converting to BD.