The early termination of clinical trials, for either benefit or harm, often generates undue enthusiasm or alarm. The enhanced publicity attending early termination of a trial promotes inappropriate interpretations that are favored by the inherent difficulty of prompt and comprehensive data review. Furthermore, the process of monitoring the accumulating outcome data for early evidence of treatment benefit or harm is fraught with many statistical and methodological difficulties. This report from a task force convened by the Working Group on Arrhythmias of the European Society of Cardiology incorporates first, a series of trials terminated appropriately or inappropriately for benefit or harm and used as examples to illustrate the importance of suitable trial design and of proper stopping rules; second, a description of the committee structure of a clinical trial; third, an analysis of the general design issues; fourth, a review of the main issues in interim analysis with special reference to main strategies for reducing the rate of false-positive claims that could result from early trial termination; and finally, a series of specific recommendations concerning the design, structure, analysis, interpretation, and presentation of a clinical trial. (Circulation. 1994;89:2892-2907.)