Palpitations-sensations of a rapid or irregular heartbeat-are most often caused by cardiac arrhythmias or anxiety. Most patients with arrhythmias do not complain of palpitations. However, any arrhythmia, including sinus tachycardia, atrial fibrillation, premature ventricular contractions, or ventricular tachycardia, can cause palpitations. Palpitations should be considered as potentially more serious if they are associated with dizziness, near-syncope, or syncope. Nonarrhythmic cardiac problems, such as mitral valve prolapse, pericarditis, and congestive heart failure, and noncardiac problems, such as hyperthyroidism, vasovagal syncope, and hypoglycemia, can cause palpitations. Palpitations also can result from stimulant drugs, and over-the-counter and prescription medications. No cause for the palpitations can be found in up to 16 percent of patients. Ambulatory electrocardiographic (ECG) monitoring usually is indicated if the etiology of palpitations cannot be determined from the patient's history, physical examination, and resting ECG. When palpitations occur unpredictably or do not occur daily, an initial two-week course of continuous closed-loop event recording is indicated. Holter monitoring for 24 to 48 hours may be appropriate in patients with daily palpitations. Trans-telephonic event monitors are more effective and cost-effective than Holter monitors for most patients.