Sleep apnoea and hypopnoea have been reported in myotonic dystrophy, but it is unclear whether this is simply attributable to the respiratory muscle weakness which is common in this condition. We therefore investigated whether breathing and oxygenation during sleep were more abnormal in patients with myotonic dystrophy than in patients with non-myotonic muscle weakness. Seven subjects were studied in each of three groups: normal controls, myotonic dystrophy and non-myotonic weakness. Patients in the latter group were chosen to represent a similar range of severity of respiratory muscle weakness to those with myotonic dystrophy. Detailed polysomnography was performed; the severity of breathing disorders during sleep was quantified in terms of the frequencies of apnoea and hypopnoea and the degree of arterial desaturation. The myotonic patients showed more frequent apnoea and hypopnoea and more severe desaturation than the other two groups; the results in the non-myotonic patients were generally intermediate. The results suggest that abnormal breathing during sleep is common in myotonic dystrophy and is not due solely to the direct effects of respiratory muscle weakness. Somnolence, which is a well recognized symptom of myotonic dystrophy, was not clearly attributable to the sleep apnoea/hypopnoea syndrome nor to abnormal sleep architecture in the myotonic patients.