Breathing patterns, ear oxygen saturation (SaO2), and EEG sleep-stage throughout an undisturbed night's sleep were compared in ten adult stable asthmatics and ten age-matched healthy subjects. The two groups slept equally long (5.0-7.2, mean 6.2 h), but the asthmatics slept less well; they had more periods of wakefulness and drowsiness and irregular breathing than did the healthy subjects. They also had greater and more frequent falls in SaO2. Most hypoxaemic episodes occurred in the rapid-eye movement phase of sleep and were associated with hypopnoea or apnoea, but no patient had a classical sleep-apnoea syndrome. The severity of nocturnal hypoxaemia was related to the level of SaO2 when the subjects were awake, but did not correlate with the fall in forced expiratory volume recorded in eight out of ten asthmatics after sleep.