Until recently, severe paradoxic responses to disopyramide have been believed to occur only, if at all, at extremely high doses. This credo has been shaken by some recent reports on severe ventricular tachyarrhythmias occurring in some patients on disopyramide. A further case (62 years, female, mitral commissurotomy in 1966, combined mitral valve lesion) is presented in whom a normal oral regimen (100 mg disopyramide four times daily) induced syncope due to ventricular tachycardia and flutter. The patient exhibited a long QT time before medication without deafness. Indication for treatment was the preservation of sinus rhythm as intermittent atrial fibrillation or flutter has been documented before. The serum potassium level was in the range of normal. Similar side-effects occurred some days later when the patient received quinidine sulfate. A survey of the most recent literature reveals a total of 20 patients with proven or suspected paradoxic responses to disopyramide. Factors that might favour the occurrence of paradoxic effects were preexisting QT prolongation, hypokaliemia, or massive overdosage. Though the total incidence of these side-effects seems to be relatively low, disopyramide should be given to special subgroups of patients only under careful monitoring.