Daytime fatigue and sleep disturbance are frequent complaints in patients with amyotrophic lateral sclerosis (ALS). However, polysomnographic data are sparse. Nocturnal respiratory insufficiency may occur despite nearly normal daytime pulmonary function. We describe the clinical presentation and polysomnographic findings in two patients with clinically and electrophysiologically confirmed ALS with minimal weakness but excessive daytime sleepiness. Polysomnography in the first patient showed a respiratory disturbance index of 43.5, and profound oxygen desaturations to 62%. The second patient had prolonged periods of hypoventilation, with oxygen saturations oscillating between 86 and 83%. Both patients showed severe sleep maintenance insomnia with a sleep efficiency < 40% and frequent arousals while asleep. Application of continuous positive airway pressure (CPAP) restored normal nocturnal ventilation, blood oxygenation and sleep parameters in the first patient; compliance, however, was poor. The second patient was unable to tolerate CPAP. We conclude that ALS patients with excessive daytime sleepiness or insomnia should undergo polysomnography to adequately diagnose nocturnal respiratory insufficiency and sleep disturbance. Compliance with treatment, however, may be poor.