Hundred and twenty-three outpatients were treated with oral cibenzoline for 3 months in order to test the efficacy and safety of the compound in the prevention of the recurrence of supraventricular arrhythmias. The dose was 260-390 mg/day for those under 70 and 130-260 mg/day for those over 70 years. All patients were converted to sinus rhythm before entry to the study and 95 patients had previously been treated with one or more drugs which had been discontinued due to lack of effect and/or poor tolerability. In 21 patients (17%) recurrence was documented by ECG or Holter monitoring, cibenzoline thus being effective in 83%. In 35 other patients there was a return of symptoms but no confirmation of recurrence. There were no relevant changes in blood pressure or heart rate. PR, QT and QTc intervals were stable but mean QRS interval increased slightly during the first week before stabilizing. Cibenzoline was discontinued because of adverse events in only 10 patients (8.1%). The most frequent complaints were nausea, vertigo and faintness. Seventy-two per cent of patients rated their well-being as "well/very well" at month one compared with 84% at month three. Cibenzoline is an appropriate first line choice in this indication.