PURPOSE: To review the evidence supporting the belief that caffeine causes cardiac arrhythmias. DATA SOURCES: Studies published since 1982 identified through computerized searches of MEDLINE, TOXLINE, and Chemical Abstracts and a review of bibliographies of relevant articles on the subject of caffeine and cardiac arrhythmias. STUDY SELECTION: All clinical studies examining caffeine as a cause of cardiac arrhythmias and a selection of basic science experiments to illustrate caffeine's effects in vitro. DATA EXTRACTION: Study quality was assessed and all available clinical data pertaining to caffeine as a cause of arrhythmias were summarized. RESULTS OF DATA ANALYSIS: In one electrophysiologic study, caffeine was associated with an increased susceptibility to provoked cardiac arrhythmias. In five placebo-controlled trials, caffeine in doses up to 500 mg daily (equivalent to 5 to 6 cups of coffee) did not increase the frequency or severity of ventricular arrhythmias. One large epidemiologic study reported an increase in the frequency of ventricular extrasystoles in persons consuming 9 or more cups of coffee daily. CONCLUSION: Moderate ingestion of caffeine does not increase the frequency or severity of cardiac arrhythmias in normal persons, patients with ischemic heart disease, or those with pre-existing serious ventricular ectopy.