Diprafenone is a new antiarrhythmic drug with a dominant local anaesthetic action and an additional beta-sympathicolytic activity. In this study, the results of long-term treatment (8 months on average) obtained from 27 patients with chronic ventricular arrhythmias are reported. Before diprafenone, all patients were treated unsuccessfully with flecainide, propafenone, sotalol, combined sotalol/flecainide and sotalol/propafenone, and another two to six antiarrhythmic agents. Following diprafenone (300-600 mg/24 h), a substantial reduction in arrhythmic activity (greater than or equal to 80%; Lown classification less than or equal to II) was achieved in 21 cases. In 12 patients, side effects (fatigue, headache, blurred vision, dizziness and heartburn) were apparent. Diprafenone had to be discontinued in five patients, because of these side effects. At dosages greater than or equal to 450 mg/24 h, the PQ interval was significantly lengthened, and QRS duration prolonged. In one patient, an AV block III degree developed. In another case, SGOT and SGPT increased significantly; this increase was reversed after the drug was discontinued. Despite these side effects, further clinical evaluation of the compound seems promising, as the antiarrhythmic potency of diprafenone is very strong and superior to that of propafenone with respect to the required doses.