Abstract::Of 1,247 consecutive patients who underwent cardiac surgery, 297 (24%) developed a post‐operative atrial tachyarrhythmia. Of these patients, 201 were suitable for treatment according to the study protocol. All patients were initially given digoxin 0.75 mg intravenously (i.v.). After two hours, those 156 patients whose atrial arrhythmias persisted were given a 2 mg/kg loading dose of disopyramide (i.v.), followed by a constant i.v. infusion (0.4 mg kg‐1tr1) or oral therapy (600 mg daily). Within a further 12 hours, 75 patients (48%) reverted to sinus rhythm, 24 within one hour. Thus 120/201 patients (60%) reverted to sinus rhythm within 14 hours of commencing therapy. Reversion rates of those patients with both atrial fibrillation and flutter (AF/AFL) were significantly lower than those with AF (p<0.001) or AFL (p<0.02)alone. A further 70 patients reverted to sinus rhythm in one to 13 (mean four) days on continued drug therapy. Elective cardioversion restored sinus rhythm in six subjects. Atrial arrhythmias persisted in five patients (2.5%) at hospital discharge. Side‐effects of disopyramide were noted in 19% of patients. Urinary retention was common (11.5%). Four patients with atrial flutter developed 1:1 atrioventricular conduction with the disopyramide loading dose. One patient with atrial fibrillation developed ventricular tachycardia during injection of the loading dose of disopyramide, but was successfully cardioverted to sinus rhythm. Two further patients developed significant hypotension (<90 mmHg systolic). Disopyramide is effective in the treatment of post‐operative atrial tachyarrhythmias, but its routine use in this situation may be associated with an unacceptably high incidence of side