The effect of orally-administered aminophylline on cardiac arrhythmias was studied in 15 patients with stable chronic obstructive pulmonary disease by continuous 24-hour ambulatory electrocardiographic recordings. During the control period, the mean frequency of ventricular ectopic beats (VEBs) per hour was 43 +/- 26 (range 0.3 to 401), and heart rate was 80 +/- 3 beats per minute. All grades of ventricular arrhythmias were seen with occasional VEBs in five patients, frequent in three, multifocal in four, coupled beats in two, and short runs of ventricular tachycardia in one patient. Seven patients had occasional atrial premature contractions, six paroxysmal atrial tachycardia, and one patient had stable atrial fibrillation. Mean frequency of VEBs per hour and heart rate were statistically similar in patients undergoing two 24-hour control recordings. Mean grade of atrial and ventricular arrhythmias also remained similar on two control recordings. After oral aminophylline, the mean frequency of VEBs per hour increased to 72 +/- 41 (P = 0.006). Heart rate increased to 88 +/- 4 beats per minutes (P = less than 0.01). The mean grade of ventricular or atrial arrhythmias remained unchanged. We conclude that orally-administered aminophylline has both arrhythmogenic and chronotropic effects, but does not change the grade of arrhythmia.