The decision to terminate a clinical trial earlier than planned is often described as ethically problematic, but it is rarely systematically analyzed as an ethical issue in its own right. This paper provides an overview of the main ethical considerations at stake in such decisions and of the main tensions between these considerations. Arguments about informed consent and the impact of early stopping on research and society are explored. We devote particular attention to a familiar conflict that arises with special urgency when early data suggest that the experimental treatment is superior. Should the trial be stopped so that participants in the control group will not be allocated a seemingly inferior treatment, or should it continue in pursuit of evidence conclusive enough to improve the care of future patients? We scrutinize three ways to address this problem. Rather than dissolving the tension, they represent different trade-offs between the respective welfare interests of subjects and future patients.