Fourteen male patients with chronic bronchitis and respiratory insufficiency were monitored during nocturnal sleep by conventional polygraphy (EEG, EOG and EMG of the chin) associated with continuous recording of the mean pulmonary arterial pressure (PAP) and, in 50% of the cases, continuous measurement of O2 saturation. Six subjects were able to sleep long enough for oxygen saturation, blood gases and PAP values to be correlated with the various phases of sleep. Frequent, significant and sometimes abrupt peaks of hypoxaemia were observed; they usually, though not necessarily, occurred during paradoxical sleep. The most pronounced nocturnal changes in SaO2 were encountered in those patients who had the most severe hypoxaemia during daytime. Nocturnal fluctuations in PAP were also pronounced (with increases of more than 20 mmHg) and often paralleled changes in SaO2. Correlation between SaO2 and PAP was good in some patients ("responders") but poor or even lacking in others ("poor-or non-responders"). This preliminary study indicates that patients with chronic bronchitis may have severe exacerbations of pulmonary arterial hypertension during nocturnal sleep and warrant nocturnal oxygen therapy.