Multifocal atrial tachycardia (MAT) was observed in 41 patients, 35 of whom were in acute respiratory distress. A hospital mortality of 37% reflected the seriousness of their underlying conditions. In no patient was the arrhythmia the primary cause of death. The MAT preceded and/or followed atrial fibrillation (AF) or atrial flutter (Af) in 19 patients (46%). The MAT simulated AF in several cases in which P-waves were inconspicuous. Our cases suggest that MAT is not a manifestation of digitalis intoxication. Quinidine sulfate, procainamide hydrochloride, lidocaine, and phenytoin (diphenythydantoin) did not affect the rhythm. Digitalis was usually not effective in slowing the ventricular response in patients with MAT. Propranolol hydrochloride was given to seven patients and was effective in slowing the atrial rate in all cases. An attempt at electrocardioversion was unsuccessful in one patient.