The recognition that certain types of ventricular premature beats, specifically repetitive forms, may be forerunners of more serious tachyarrhythmias has led to the practice of prophylactic therapy with antiarrhythmic drugs to suppress these forms in patients who have underlying cardiac disease. Several antiarrhythmic drugs are available for oral therapy, and many others are undergoing investigation. The usefulness of these agents is often limited by frequent side effects, which include idiosyncratic and unpredictable reactions that are not related to drug level. One such effect is the aggravation of the arrhythmia being treated. Noninvasive drug testing in 245 patients for a total of 1024 single drug tests showed 113 studies (11 per cent) to have been aggravated by the antiarrhythmic drug. At least one drug was proarrhythmic in 78 patients (32 per cent). Aggravation of arrhythmia is a potentially serious drug complication that occurs with all antiarrhythmic agents.