Obstructive sleep apnoea (OSA) is a sleep-related breathing disorder with occurrence of partial or complete closure of the upper airway during sleep, despite ongoing effort to breathe. OSA has a high prevalence for cardiovascular disease, hypertension, and stroke. Patients face cyclical deoxygenation/re-oxygenation changes resulting in episodic hypercapnia and hypoxia. Events are graded by the apnoea-hypopnea index (AHI) which is the sum of apnoea and hypopnoea events. The severity of OSA has been linked to various factors: body characteristics, upper respiratory tract muscle activity, chemosensitivity, as well as to cytokines. The contribution of chemosensitivity to the severity of OSA and its connection to body characteristics is not clear. Consequently, we investigated newly diagnosed, untreated OSA patients (n=48) and assessed ventilator responses to breathing normoxic carbon dioxide (6%) gas mixture, oxygen (13%), and combined carbon dioxide (6%) with oxygen (13%) mixture (Earing et al. 2016) to assess chemosensitivity. Moreover, body characteristics, lung function, and blood parameters adiponectin, CRP, leptin, and endocannabinoids (AEA and 2AG) were measured. In addition, a group of healthy people (n=49) were assessed on all parameters, except blood factors, for comparison with OSA patients. In OSA patients, AHI was significantly correlated with ventilator response to carbon dioxide (6%), r=-0.511, p=0.001; there was no significant association with response to hypoxic gas breathing. Additionally, there were significant correlations of AHI with body characteristics, in particular with neck circumference (r=0.509, p=0.001). Blood parameters showed no significant correlations with AHI. Healthy participants, revealed significant differences in all measured parameters. To investigate whether reduced chemosensitivity to carbon dioxide was likely to be a contributing factor of body characteristics, we performed multiple regression analysis for prediction of carbon dioxide sensitivity variance in the whole participant group. Backward methods revealed that neck circumference was best predictor, explaining about 30% (r2=0.286, p<0.0001, n=87) of carbon dioxide response variance. Conclusion: AHI is strongly associated with carbon dioxide sensitivity and body characteristics in OSA patients; however, only a smaller proportion of the altered CO2 sensitivity can be attributed to selected body characteristics. It is likely that other than simply mechanical factors influence the alteration of chemosensitivity in OSA.