Objective: Aggression is common and challenging in psychiatric emergency departments (PED). However, the prevalence of aggression and its correlates in PED patients are not well documented. This study compared the prevalence of aggression between patients with acute schizophrenia and manic episodes. Methods: In this cross-sectional study, patients at a psychiatric emergency department were assessed with measurements of aggression, psychotic and manic symptoms. Results: A total of 4,172 patients were included. The prevalence of aggression was 54.8% (95%CI=53.3%-65.2%) in the whole sample, with 48.0% (95%CI=45.8%-50.1%) in patients with an acute schizophrenia episode, and 61.8% (95%CI=59.8%-63.9%) in patients with a manic episode. Multiple logistic regression analysis revealed that, within the acute schizophrenia episode group, male gender (OR=1.47, P<0.01), involuntary admission (OR=3.61, P<0.01) and more severe manic symptoms (OR=1.30, P<0.01) were significantly associated with aggression. Within the manic episode group, living in Beijing (OR=1.51, P<0.01), unemployment (OR=1.34, P=0.03), involuntary admission (OR=7.93, P<0.01), lower education (OR=0.95, P=0.01) and more severe psychotic symptoms (OR=1.05, P<0.01) were significantly associated with aggression. Conclusion: In this study, aggression appeared to be more common among patients with a manic episode than those with an acute schizophrenia episode. Considering the significant risk of aggression on psychiatric emergency care, appropriate and effective management of aggression in this population group need to be developed.