Marcus EH Ong1, Faith SP Ng2, Susan Yap1, Kok Leong Yong1, Mary A Peberdy3, Joseph P Ornato41Department of Emergency Medicine, Singapore General Hospital, Singapore; 2Clinical Trials and Epidemiology Research Unit (now known as Singapore Clinical Research Institute), Singapore; 3Division of Cardiology, Virginia Commonwealth University – Medical College of Virginia, Richmond, VA, USA; 4Department of Emergency Medicine, Virginia Commonwealth University – Medical College of Virginia, Richmond, VA, USAObjective: We aimed to determine whether there is a seasonal variation of out-of-hospital cardiac arrests (OHCA) in an equatorial climate, which does not experience seasonal environmental change.Methods: We conducted an observational prospective study looking at the occurrence of OHCA in Singapore. Included were all patients with OHCA presented to Emergency Departments across the country. We examined the monthly, daily, and hourly number of cases over a threeyear period. Data was analyzed using analysis of variance (ANOVA).Results: From October, 1st 2001 to October, 14th 2004, 2428 patients were enrolled in the study. Mean age for cardiac arrests was 60.6 years with 68.0% male. Ethnic distribution was 69.5% Chinese, 15.0% Malay, 11.0% Indian, and 4.4% Others. There was no significant seasonal variation (spring/summer/fall/winter) of events (ANOVA P = 0.71), monthly variation (P = 0.88) or yearly variation (P = 0.26). We did find weekly peaks on Mondays and a circadian pattern with daily peaks from 9–10 am.Conclusions: We did not find any discernable seasonal pattern of cardiac arrests. This contrasts with findings from temperate countries and suggests a climatic influence on cardiac arrest occurrence. We also found that sudden cardiac arrests follow a circadian pattern.Keywords: cardiopulmonary resuscitation, cardiac arrest, seasonal pattern, circadian pattern