Artifacts mimicking a variety of dysrhythmias occur relatively frequently in ambulatory ECG (Holter) monitoring records. Proper interpretation is imperative if serious therapeutic errors are to be avoided. Two-channel recording systems may facilitate recognition of some, but not all, of these artifacts. Pseudodysrhythmias may mimic paroxysmal supraventricular tachycardia and atrial fibrillation, atrial dissociation, extrasystoles, and sinus pauses. There are several causes of pseudodysrhythmias. Failure to recognize these patterns may result in serious errors in patient management.