Five patients with severe kyphoscoliosis were studied during sleep. A spectrum of breathing abnormalities was found ranging from no abnormalities to severe episodes of prolonged central apnea. Rapid-eye-movement (REM) sleep was the period of greatest physiologic disturbance in all the patients, being the time of greatest oxygen desaturation. Within this small group, those with the most clinical evidence of chronic hypoxemia, polycythemia, and cor pulmonale, had the most severe derangements during sleep. Derangements in breathing pattern and arterial oxygen saturation (SaO2) had no apparent relation to the degree of thoracic deformity, the results of pulmonary function tests, arterial PCO2, or the chemical drives to breathe.