Purpose Acute polymorphic psychotic disorder (APPD) without symptoms of schizophrenia is a serious psychiatric condition. APPD can be effectively treated with electroconvulsive therapy (ECT), but little is known about relapse prevention after ECT for APPD. Materials and methods This was a retrospective register-based study conducted with 97 patients with APPD (ICD-10 diagnosis F23.0). We estimated the rates of readmission and suicide, and the prognostic factors of these outcomes after ECT. We combined data from several national Swedish registers and used Coxs regression analysis to identify demographic factors, disease characteristics, and relapse preventive treatments that predicted time to readmission or suicide (relapse). Data registered between 2011 and 2016 were used in the study. Results Twenty percent of cases relapsed within a year. Thereafter, relapse rate was low. Two cases died during follow-up, whereof one by suicide. Anxiolytic treatment, lamotrigine treatment, and having more than four previous psychiatric hospital admissions were associated with shorter time to relapse. The most robust of these associations was between anxiolytics and relapse risk. Conclusions The first year after discharge from APPD is the period associated with the highest risk of relapse. Having many previous admissions was associated to relapse risk after ECT for APPD. The associations between anxiolytics, lamotrigine, and relapse are uncertain and might be influenced by indication bias.