Recent epidemiological data indicate that obstructive sleep apnoea (OSA) and related conditions are extremely common in the middle-aged population. Obesity is an important aetiological factor in sleep-disordered breathing with a multifactorial role in the pathogenesis of upper airway occlusion. One extreme of the spectrum of sleep-disordered breathing is obesity-hypoventilation syndrome (one type of OSA with awake respiratory failure). Sleep-disordered breathing has a number of clinical consequences, including excess cardiovascular morbidity. Obesity is an important confounder of this association. Treatment of these disorders has been revolutionized by the use of nasal continuous positive airway pressure (CPAP). Weight reduction reduces apnoea severity but is not curative in most obese patients with sleep apnoea.