BACKGROUND: Many physicians hesitate to discuss do-not-resuscitate (DNR) orders with patients or family members in critical situations. In the intensive care unit (ICU), delayed DNR decisions could cause unintentional cardiopulmonary resuscitation, patient distress, and substantial cost. We investigated whether the timing of DNR designation affects patient outcome in the medical ICU. METHODS: We enrolled retrospective patients with written DNR orders in a medical ICU (13 bed) from June 1, 2014 to May 31, 2015. The patients were divided into two groups: early DNR patients for whom DNR orders were implemented within 48 h of ICU admission, and late DNR patients for whom DNR orders were implemented more than 48 h after ICU admission. RESULTS: Herein, 354 patients were admitted to the medical ICU and among them, 80 (22.6%) patients had requested DNR orders. Of these patients, 37 (46.3%) had designated DNR orders within 48 hours of ICU admission and 43 (53.7%) patients had designated DNR orders more than 48 hours after ICU admission. Compared with early DNR patients, late DNR patients tended to withhold or withdraw life-sustaining management (18.9% vs. 37.2%, p = 0.072). DNR consent forms were signed by family members instead of the patients. Septic shock was the most common cause of medical ICU admission in both the early and late DNR patients (54.1% vs. 37.2%, p = 0.131). There was no difference in in-hospital mortality (83.8% vs. 81.4%, p = 0.779). Late DNR patients had longer ICU stays than early DNR patients (7.4 ± 8.1 vs. 19.7 ± 19.2, p < 0.001). CONCLUSIONS: Clinical outcomes are not influenced by the time of DNR designation in the medical ICU. The late DNR group is associated with a longer length of ICU stay and a tendency of withholding or withdrawing life-sustaining treatment. However, further studies are needed to clarify the guideline for end-of-life care in critically ill patients.