Pirmenol, a new class IA antiarrhythmic agent, has shown promise in short-term trials, but long-term efficacy has not been documented. We thus evaluated 11 patients with frequent (greater than or equal to 60/h) premature ventricular complexes (PVC) given oral pirmenol for 25-727 days. Ten of 11 patients entering the long-term open trial had shown greater than or equal to 70% (mean 83%) PVC suppression during in-hospital pirmenol dose ranging. Long-term pirmenol was given in divided doses of 100-600 mg/day. Mean PVC frequency during baseline was 13,078/24 h (range, 3,218-32,718); couplets averaged 481/24 h (1-2,829) and runs 45/24 h (0-334). Ambulatory monitoring was performed at 1, 3, 6, and 12 months, then semiannually. Mean absolute PVC suppression at 1 month averaged 75% (p less than or equal to 0.02). Median individual percentage PVC suppression was 94%. During the first 3 months, 8 patients (73%) continued to show a favorable response (greater than or equal to 70% suppression), and 3 had arrhythmia recurrence and were dropped. One responder was withdrawn after the onset of paroxysmal atrial fibrillation, and another early responder was withdrawn after 3 months because of arrhythmia relapse. Six patients have been treated for over 1 year, with 99% mean PVC suppression. Mean couplet and run frequencies at 1 month decreased by means of 76% (p less than or equal to 0.05) and 92% (p = 0.001) respectively. At 1 year, couplets were suppressed 99.8% and runs by 99.7% in the 6 patients remaining on pirmenol.(ABSTRACT TRUNCATED AT 250 WORDS)