Introduction: Early prediction of the outcome in patients with cardiopulmonary arrest (CPA) is important to select the optimal management. The pupil diameter is a simple neurological examination and it predicts the prognosis in patient with stroke. However, the clinical significance of pupil diameter in CPA has not been elucidated. Hypothesis: We hypothesized that pupil diameter would predict the prognosis of CPA patients. Methods: We retrospectively analyzed consecutive 45 patients with CPA and return of spontaneous circulation (ROSC) after cardiopulmonary resuscitation in our hospital. We analyzed age, sex, cause of CPA, time during CPA, pupil diameter at admission and death in hospital. Results: The mean of age was 67±20 years and male were 34 (76%). The median of CPA time was 33 min [25%, 75%; 22, 46] and pupil diameter was 4.8 mm [3.0, 6.0]. The 26 (58%) patients died in hospital after ROSC. There was no significant difference in age and sex between survival (n=19) and dead (n=26). The CPA time was significantly shorter in survival than that in dead (21 min [10, 39] vs 30 min [30, 52], p=0.005). Furthermore, pupil diameter was significantly smaller in survival compared with dead (3.0mm [2.5, 4.3] vs 5.0mm [4.4, 6.0]) (Figure). The receiver-operating characteristic (ROC) curve for survival after ROSC demonstrated that the area under curve was 0.73 and provided an optimal cut-off value at 4.0mm in pupil diameter with 75% sensitivity and 75% specificity. When CPA time was 28 minutes, the area under curve was 0.76, and sensitivity and specificity were 78% and 71%, respectively. Conclusion: As same as CPA time, pupil diameter is suggested to predict the outcome after ROSC in CPA patients.