Carbon monoxide (CO) poisoning remains a common cause of both suicidal and accidental deaths in the United States. As a consequence, determination of the percent carboxyhemoglobin (%COHb) level in postmortem blood is a common analysis performed in toxicology laboratories. The blood specimens analyzed are generally preserved with either EDTA or sodium fluoride. Potentially problematic scenarios that may arise in conjunction with CO analysis are a first analysis or a reanalysis requested months or years after the initial toxicology testing is completed; both raise the issue of the stability of carboxyhemoglobin in stored postmortem blood specimens. A study was conducted at the Bexar County Medical Examiner's Office to evaluate the stability of CO in blood samples collected in red-, gray-, and purple-top tubes by comparing results obtained at the time of the autopsy and after two years of storage at 3 degrees C using either an IL 282 or 682 CO-Oximeter. The results from this study suggest that carboxyhemoglobin is stable in blood specimens collected in vacutainer tubes, with or without preservative, and stored refrigerated for up to two years.