BACKGROUND: To improve survival rate, the main focus of adult cardiac arrest management includes rapid recognition, prompt administration of cardiopulmonary resuscitation (CPR), defibrillation for shockable rhythms, post-return of spontaneous circulation (ROSC) care, and identification and treatment of underlying causes. This study aimed to identify the determinants of unrecognized cardiac arrest, and to study the recognition rate of out-of-hospital cardiac arrest (OHCA) by emergency medical services call handlers. METHODS: We included OHCA patients who were transferred to hospital via Emergency Medical Services (EMS) of Srinagarind hospital, Khon Kaen, Thailand, from 1st January 2020 to 31st December 2020. The primary outcome was to identify symptoms that lead to an unrecognized cardiac arrest by the EMS call handlers. Secondary outcomes were to identify the recognition rate of OHCA by emergency medical services call handlers, and assess the outcome of CPR performed on OHCA patients. RESULTS: There were a total of 58 patients in the present study, 26 patients (44.8%) and 32 patients (55.2%) belonged to the unrecognized and recognized cardiac arrest groups, respectively. The most common symptoms that led to unrecognized cardiac arrest were a state of unconsciousness (46.2%), major trauma (15.4%), and seizure-like activity (11.5%). The rate of ROSC was higher in the unrecognized cardiac arrest group (34.6% vs. 15.6%) but the rate of survival to hospital discharge was higher in the recognized cardiac arrest group (6.3% vs 0%). CONCLUSIONS: Falling unconscious is the most common symptom of unrecognized OHCA cases seen by EMS in Thailand. Basic life support, especially an immediate assessment of a patient’s respiratory status should be taught in health programs in school or through public service channels.