Hypoxemia, obstructive apnea, and disordered breathing during sleep have been documented in adult patients with chronic obstructive pulmonary disease. We studied sleep, oxygen saturation, and breathing in nine patients (ages 17 to 26 years) with cystic fibrosis (CF) lung disease by means of all-night polygraphic tracings. Two had brief episodes of oxygen desaturation during rapid eye movement (REM) sleep, and one patient with severe disease had prolonged desaturation with episodic worsening associated with disordered breathing in REM sleep and with cough. No patient had apnea or any evidence of upper airway obstruction, in contrast with studies of older patients. In three patients, nocturnal coughing contributed to disruption of sleep cycles. Sleep oxygen desaturation may contribute to the development or progression of cor pulmonale in some patients with CF, but it seems to be most severe only in those with advanced disease.