BACKGROUND: The initial management of acute critical patients is important. However, not all hospital facilities and staff are available during off-duty time. We determined the effects of intensive care unit (ICU) admission time on patient outcomes. METHODS: This retrospective cohort study was conducted in a 28-bed medical ICU in 1 tertiary university hospital. Patients who were admitted between 1 March 2009 and 31 August 2009 were divided according to the time of admission into the "duty time group" (9 AM-5 PM on weekdays) and the "off-duty time group" (5 PM-9 AM on weekdays and at any time on weekends). The baseline characteristics and clinical outcomes were compared between these two groups. The primary endpoint of this study was hospital mortality; the secondary endpoints were ICU mortality and length of ICU stay, hospital length of stay, and mechanical ventilation time. RESULTS: Two hundred eight (64.8%) of 321 enrolled patients were admitted during off-duty time. The baseline characteristics between the 2 groups were not significantly different. Hospital mortality was 37 (32.7%) in the "duty time group" and 82 (38.4%) in the "off-duty time group" (p = 0.237). There were no significant differences in secondary endpoints between the two groups. CONCLUSIONS: Off-duty time admission to the ICU had no effect on hospital and ICU mortality, length of hospital and ICU stay, and mechanical ventilation time compared to duty time admission.