Abstract
Data Safety Monitoring Committees (DSMB) for large, long-term randomized trials of agents in common use face challenging problems especially when the emerging data indicate unanticipated effects. The DSMB for the Women's Health Initiative Clinical Trials, on observing early indication of a surprising adverse cardiovascular effect of post-menopausal hormones, spent several years deliberating what recommendations it should make. This paper describes the dilemmas faced by the DSMB and the considerations it made over the course of its existence. The paper concludes with some recommendations for other DSMBs.