Apneas generally reappear in patients with obstructive sleep apnea (OSA) when treatment with continuous positive airway pressure (CPAP) is interrupted. However, a single-night treatment interruption may be associated with a lesser severity of sleep apneas than before treatment. We hypothesized that this decrease in severity of sleep apneas reflects changes in the respiratory response to upper airway obstruction. Therefore, we compared indexes of respiratory effort during sleep in 25 patients with OSA before and after 1 year of CPAP treatment. Respiratory effort was assessed by means of an esophageal balloon. After 1 year of CPAP treatment, there was a decrease in the maximal end-apneic esophageal pressure swings (Pes) (from 56.7 +/- 5.4 to 30.3 +/- 2.6 cm H2O; p = 0.000; mean +/- SEM), in the overall increase in Pes during an apnea (35.2 +/- 3.6 vs 16.5 +/- 1.5 cm H2O; p = 0.000), as well as the rate of increase in Pes (1.1 +/- 0.1 vs 0.6 +/- 0.1 cm H2O/s; p = 0.000). Although body mass index (BMI) did not change significantly, the individual changes in BMI significantly correlated with the changes in respiratory effort after 1 year of CPAP treatment. Apnea duration and apnea-related oxygen desaturation also decreased significantly. We conclude that long-term CPAP treatment induces changes in respiratory control that persist at least on the first night of treatment interruption.