Background: The treatment of choice for cycloid psychosis has traditionally been electroconvulsive therapy (ECT), but there is a lack of studies on its effectiveness.Aims: The primary aim of this register study was to determine the rates of remission and response after ECT for cycloid psychosis. The secondary aim was to examine possible predictors of outcome.Methods: Data were obtained from the National Quality Register for ECT in Sweden. The study population was patients (n=42) who received ECT for acute polymorphic psychotic disorder without symptoms of schizophrenia or for cycloid psychosis between 2011-2015 in 13 hospitals. Remission and response rates were calculated using Clinical Global Impression-Severity (CGI-S) and -Improvement scores, respectively. Variables with possible predictive value were tested using Chi-square and Fishers exact test.Results: The response rate was 90.5%. The remission rate was 45.2%. Of 42 patients, 40 improved their CGI-S score after ECT (pamp;lt;0.001). The mean number of ECT treatments was 2.5 for non-responders and 7.0 for responders (p=0.010). The mean number of ECT treatments did not differ significantly between remitters and non-remitters (7.2 vs 6.1, p=0.31). None of the other investigated potential predictors was statistically significantly associated with outcome.Conclusions: ECT is an effective treatment for cycloid psychosis. Future studies need to compare the outcome of ECT to that of other treatment strategies. Clinical implications: The high response rate with ECT indicates that cycloid psychosis is a clinically useful diagnosis.