Different aspects of mechanical loading were analyzed in a group of 25 patients with chronic obstructive pulmonary disease (COPD) who all had severe expiratory airflow limitation but different arterial CO2 values. It was found that the maximal expiratory flow rates (V75, V50, V25, VFRC), functional residual capacity (FRC), residual volume (RV), total lung capacity (TLC), specific airways conductance (SGaw) measured during inspiration or expiration, and lung recoil pressure (PLel) were not correlated to the PaCO2. Only five parameters were significantly linked to the PaCO2; they were the peak inspiratory flow (PIF) (r = -0.57, p < 0.002), the inspiratory vital capacity (IVC) (r = -0.46, p < 0.01), the maximal voluntary ventilation (MVV) (r = -0.49, p < 0.01), the total SGaw (r = -0.40, p < 0.03), and the forced expiratory flow volume in one second (FEV1) (r = -0.36, p < 0.05). It was concluded that the airflow limitation during the inspiratory phase has an important contribution to the CO2 retention in patients with COPD, and therefore analysis of both phases of the respiratory cycle is necessary to assess the severity of airway obstruction in this disease.