To assess the relative contributions of age, gender, obesity, pulmonary function, and the severity of sleep-induced respiratory abnormalities to the development of alveolar hypoventilation in patients with occlusive sleep apnea syndrome, prospective data from III patients with occlusive sleep apnea were analyzed by stepwise logistic and multiple regression techniques. The significant variables in a logistic regression model predicting the presence of hypercapnia were daytime arterial oxygen pressure (PaO2; p less than 0.0001) and gender (p less than 0.04), the latter reflecting the higher number of hypercapnic women in our patient population. Multiple regression analysis performed in the hypercapnic group to study the determinants of the severity of elevation of arterial carbon dioxide tension (PaCO2) revealed significant contribution from the PaO2, the apnea-plus-hypopnea index (AHI), and the percent predicted forced vital capacity (r2 = 0.56; p less than 0.0001), whereas in the normocapnic patients, PaCO2 related to PaO2 only. These results suggest that daytime hypoxemia, mechanical impairment of the respiratory system due to obesity or obstructive airway disease (or both), and the severity of sleep-induced respiratory abnormalities as assessed by AHI contribute to the severity of carbon dioxide retention in patients with occlusive sleep apnea in a multifactorial fashion.