The aim of Mobile Coronary Care Units (M.C.C.U.) is to reduce the delay in delivering intensive care to patients with a heart attack. In the city of Florence a M.C.C.U. has been available since November 1979. During the first year the staff of the M.C.C.U. has treated 158 cases of serious cardiac arrhythmias which occurred among 486 interventions. In 94 patients cardiac arrhythmias followed an acute coronary attack. In 64 patients coronary heart disease could not be demonstrated. This study concerns the latter group of patients. The mean age was 65.2 years and 39 patients (61%) were women. The mean time from the onset of the symptoms to the arrival of the M.C.C.U. team was 3h and 2 min, whereas the mean time from the call to the arrival was 14 min. Sixty patients had atrial arrhythmias (29 atrial fibrillation, 2 atrial flutter, 22 atrial tachycardia, 7 premature atrial contractions) and 4 patients had ventricular arrhythmias (1 ventricular tachycardia, 1 ventricular flutter, 2 premature ventricular contractions). In thirty-nine patients (61%) the cardiac arrhythmia was abolished by the staff of the M.C.C.U.. Of the remaining 28 patients, 10 were brought to the hospital and 18 were left at home. None of these needed later admission to the hospital. So the treatment at home of cardiac arrhythmias has been successful in the majority of patients. Bunaftine was the antiarrhythmic drug more frequently used (23 cases, 34%) with a high percentage of success (87%). In planning medical emergency services to the community, one can envisage the use of the M.C.C.U. facilities to treat at home those arrhythmias that are not associated with an acute coronary attack.