While maximal exercise testing is useful for detection of arrhythmias and assessment of antiarrhythmic drug efficacy, few reports have documented the safety of this procedure in patients with malignant ventricular arrhythmias. We reviewed the complications of symptom-limited exercise in 263 patients with such arrhythmias who underwent a total of 1377 maximal treadmill tests. Seventy-four percent of the population studied had a history of ventricular fibrillation or hemodynamically compromising ventricular tachycardia and the remainder had experienced ventricular tachycardia in the setting of either recent myocardial infarction or poor left ventricular function. A complication was defined as the occurrence of arrhythmia during exercise testing--ventricular fibrillation, ventricular tachycardia, or bradycardia--that mandated immediate medical treatment (cardioversion, use of intravenous drugs, or closed-chest compression). Complications were noted in 24 patients (9.1%) during 32 tests (2.3%), whereas 239 patients (90.9%) were free of complication during 1345 tests (97.7%). There were no deaths, myocardial infarctions, or lasting morbid events. Clinical descriptors associated with complications included male sex, presence of coronary artery disease, and a history of exertional arrhythmia (p less than .05). Clinical variables previously considered to confer increased risk during exercise, such as poor left ventricular function, high-grade ventricular arrhythmias (Lown grade 4A or 4B) before or during exercise, exertional hypotension, and ST depression, were not predictive of complications (p greater than .05). Occurrence of a complication was also unaffected by the use of antiarrhythmic drugs at the time of exercise (chi square = 0.19, p greater than .05).(ABSTRACT TRUNCATED AT 250 WORDS)