BACKGROUND Emergency department (ED) overcrowding is a severe health care concern, while anxiety and depression rates among ED patients have been reported to be substantially higher compared to the general population. We hypothesized that anxiety due to over crowdedness may lead to adverse events in EDs. AIM To investigate correlations between crowdedness in EDs and anxiety of patients and nurses, and to identify factors affecting their anxiety. METHODS In this prospective observational study, a total 43 nurses and 389 emergency patients from two tier III hospitals located in Beijing were included from January 2016 to August 2017. Patients were grouped into inpatients when they were hospitalized after diagnoses, or into outpatients when they were discharged after treatments. The State Trait Anxiety Inventory (STAI Form Y) questionnaire was used to investigate patient and nurse anxieties, while crowdedness of EDs was evaluated with the National Emergency Department Over Crowding Score. RESULTS The present results revealed that state anxiety scores (49.50 +/- 6.00vs50.80 +/- 2.80,P= 0.005) and trait anxiety scores (45.40 +/- 5.70vs46.80 +/- 2.70,P= 0.002) between inpatients (n= 173) and outpatients (n= 216) were significantly different, while the state anxiety of nurses (44.70 +/- 5.80) was different from those of both patient groups. Generalized linear regression analysis demonstrated that multiple factors, including crowdedness in the ED, were associated with state and trait anxieties for both inpatients and outpatients. In addition, there was an interaction between state anxiety and trait anxieties. However, multivariable regression analysis showed that while overcrowding in the ED did not directly correlate with patients' and nurses' anxiety levels, the factors that did correlate with state and trait anxieties of inpatients were related to crowdedness. These factors included waiting time in the ED, the number of patients treated, and the number of nurses in the ED, whereas for nurses, only state and trait anxieties correlated significantly with each other. CONCLUSION Waiting time, the number of patients treated, and the number of nurses present in the ED correlate with patient anxiety in EDs, but crowdedness has no effect on nurse or patient anxiety.