Carbon monoxide (CO) is a toxic gas and it is the leading cause of lethal poisonings worldwide. Common sources include vehicle exhaust, household fires and improperly maintained heating systems. CO is binding to hemoglobin thus preventing the oxygen transport. The stable complex is called carboxyhemoglobin (COHb) and it can be measured in ante- or postmortem blood specimen, using spectrophotometry or gas chromatography. COHb blood levels expressed in the percentage of the whole hemoglobin. If the amount is exceeding 50%, it’s considered life- threatening. The aim of this study is the long-term retrospective examination of the poisoning cases between 1982 and 2022. Another aim is to prove that the presence of some preexisting pathology can lead to a lower lethal concentration of COHb (<50%). In this 40 year period we examined cases of death caused by CO poisoning in Hajdú-Bihar and Szabolcs-Szatmár-Bereg County. For each case the gender, age of the deceased, along with the time and place of their death, the source of CO and the preexisting pathology, autopsy findings and toxicology results (COHb, alcohol) were collected from the autopsy register of The Institute of Forensic Medicine, Debrecen. Statistical analysis was performed using the Excel software (Chi-squared test with Yates's correction, T-test). A total of 414 toxicological tests were requested to measure COHb levels. The focus group includes 291 cases. The cause of death was carbon-monoxide poisoning in 186, smoke poisoning in 50, and burn shock in 55 of the cases. Among them were 195 male and 96 female (average age: 52.7 years ± 21.74). January (13.4%), February (15.8%) and December (20.6%) had the highest number of fatal cases. The majority of them were caused by accident. In the 1980s and 1990s the number of deaths were higher in Debrecen, then it decreases. In 2008, the jurisdiction territory of the Institute has expanded, since then the majority of cases have come from smaller settlements. Concerning the comorbidities, (heart failure, cardiomyopathy, respiratory diseases, arteriosclerosis, alcohol, tumor) the statistical analysis shows that most comorbidities contribute to a lower, but more than 50% COHb level. We found that only heart failure is associated with lethal levels of COHb below 50% (Chi-squared: p=0.000128, T-test: p=0.00000001401542). Even now, CO poisoning is a relevant problem. Public education about prevention, recognition and providing emergency treatment is needed.