Ambulatory electrocardiograms obtained in 69 patients enrolled in the nocturnal oxygen therapy trial group were examined for frequency and significance of arrhythmias in patients with stable chronic obstructive lung disease. Ventricular premature beats occurred in 83 percent, ventricular bigeminy in 68 percent, paired ventricular premature beats in 61 percent, and nonsustained ventricular tachycardia in 22 percent of the patients. Supraventricular tachycardia occurred in 69 percent. Repetitive ventricular arrhythmia occurred in 64 percent of the patients, and was significantly more frequent in men and in patients with edema or elevated PCO2. Ventricular premature beats greater or equal to 25 per hour occurred in 35 percent of the patients. Univariate and multivariate Cox proportional hazards analysis showed that a history of coronary heart disease, increased sinus heart rate and decreased maximum work load (measured by maximal treadmill exercise test)--but not arrhythmias--were predictors of death.