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Because most medical staff work from Monday–Friday, it is possible that they are relatively more fatigued and less capable of providing emergency supportive services on Thursday–Sunday (Thu–Sun) than on Monday–Wednesday (Mon–Wed). In this study, we aimed to analyze the incidence of in-hospital cardiopulmonary resuscitation (CPR) to determine if it differed between Thu–Sun and Mon–Wed.This retrospective observational study of in-hospital CPR was performed during 2012 to 2016 among inpatients at the Seoul National University Bundang Hospital. The primary outcome was the incidence of in-hospital CPR per 1000 inpatients in the Mon–Wed and Thu–Sun periods. Secondary outcomes included differences in the CPR incidence by time of day and season.In the study, 1195 cases of in-hospital CPR were included. The incidence of in-hospital CPR per 1000 inpatients was significantly higher on Thu–Sun (mean: 0.595, 95% confidence interval [CI]: 0.564–0.626) than on Mon–Wed (mean: 0.505, 95% CI: 0.474–0.536, P < .001). There were no seasonal variations in the incidence of in-hospital CPR. However, in-hospital CPR was most frequently performed between 16:00 and 24:00, and the return of spontaneous circulation (ROSC) rate was the lowest among cases that occurred between 0:00 and 8:00. In addition, the ROSC rate was lowest among female patients, patients with cardiac arrest, and after in-hospital CPR performed on a Sunday.The incidence of in-hospital CPR per 1000 inpatients was significantly higher on Thu–Sun than on Mon–Wed. No seasonal variations were observed in the incidence of in-hospital CPR, but the data suggest circadian variations and differences in ROSC rates.