OBJECTIVES: Obesity hypoventilation syndrome (OHS) and some neuromuscular diseases (NMD) present with hypercapnic respiratory failure. Arterial blood gas (ABG) analysis is important in the diagnosis, follow-up, and treatment response of these diseases. However, ABG sampling is difficult in these patients because of excessive subcutaneous fat tissue, muscle atrophy, or contracture. The aim of this study is to investigate the value of venous blood gas (VBG), which is an easier and less complicated method, among stable patients with OHS and NMD. METHODS: The study included stable OHS and NMD patients who had been previously diagnosed and followed up between March 2017 and May 2017 in the outpatient clinic. ABG was taken from all patients in room air, and peripheral VBG was taken within 5 min after ABG sampling. RESULTS: Thirty-six patients with OHS and 46 patients with NMD were included in the study. There was a moderate positive correlation between arterial and venous pH values for all patients (rs= 0.590, P < 0.001). There were a strong and very strong positive correlations between arterial and venous pCO2and HCO3values (rs= 0.725 and rs= 0.934, respectively) (P < 0.001). There was no correlation between arterial and venous pO2and saturation values. There was an agreement in BlanduAltman method for the values of ABG and VBG (pH, pCO2, and HCO3). CONCLUSIONS: There was a correlation between ABG and VBG values (pH, pCO2, and HCO3). VBG parameters (pH, pCO2, and HCO3) can be used safely instead of ABG parameters which have many risks, during treatment and follow-up of patients with OHS and NMD.