Carbon Monoxide (CO) poisoning is one of the most common causes of death in Iran. The present study investigated the indoor and outdoor CO concentrations at hospitals under the supervision of Mazandaran University of Medical Sciences in Sari, Iran.Indoor and outdoor measured CO were conducted over a period of one month during the 2012 winter season in 4 hospital kitchens using a direct measurement device specialized for CO. The average concentrations of CO gas through gas measuring stations were considered at three stations both in and out of the kitchens.The highest and the lowest average concentrations of outdoor CO in Fatemeh Zahra Hospital (about 2 ppm) and Zare Hospital (about 0.33 ppm) were measured respectively. Besides, the highest and the lowest average concentrations of indoor CO were detected in Imam Hospital (as 9.66 ppm) and BuAli Hospital (as7.66 ppm). The highest ratios of I/O (approximately 10 ppm) have been conducted between 8:00 and 9:00a.m. The lowest median concentrations of CO (approximately 8 ppm) between 11:00 and 12:00 a.m. have been measured. Concentrations of CO in all hospitals in this study were significant.This fact shows poorly ventilation of kitchen due to lack of ventilation design engineering.