The electrocardiograms of 80 ambulatory patients receiving antiarrhythmic therapy were supervised with the help of a transtelephone monitoring system. The patients used a pocket-size modulator and reported several times daily to the receiving center integrated into the intensive cardiac care unit. The surveillance lasted for 5-28 days during which various drugs in varying dosages were administered to suppress or prevent dysrhythmias. In 94% of the patients, a satisfactory therapeutic achievement was obtained. The transtelephone system provides easy diagnosis, immediate pattern recognition, direct and frequent contact with the ambulatory patient and long periods of follow-up. During this time, the proper antiarrhythmic agent can successfully be defined, its effective dose can safely be determined and unnecessary hospitalization can thus be prevented.