AIMS: This article describes a challenging data monitoring experience that occurred in a major international randomized placebo-controlled trial in patients with heart failure, in which the accumulating interim data showed an excess of deaths on the active treatment. METHODS AND RESULTS: The MOXonidine CONgestive Heart Failure trial was a randomized comparison of moxonidine, a central sympathetic inhibitor, with placebo. It was planned to recruit 4500 patients with heart failure. The primary endpoint was all-cause mortality, and average follow-up was anticipated to be around 2.5 years until 724 deaths occurred. The trial Data Monitoring Board (DMB) was to conduct safety monitoring reviews of interim data at least every six months, and make their recommendations to the Executive Committee. Within a few months of the study starting, the Data Monitoring Board (DMB) observed an emerging trend of an excess of deaths on moxonidine compared with placebo. This article describes the sequence of events that unfolded: several DMB meetings to evaluate the accumulating evidence, a DMB recommendation to stop the trial, consequent dialogue with the Executive Committee and sponsor leading to a final decision to stop the trial. Ten months after the first patient was randomized, the study was stopped based on 46 versus 25 deaths in 990 moxonidine and 943 placebo patients, respectively, P=0.01. The final published evidence had 54 versus 32 deaths, P=0.012. CONCLUSIONS: This study illustrates the problems faced by a DMB, and subsequently the trial Executive Committee and sponsor, in deciding how to act in the face of an emerging (and agonizing) negative trend for mortality in a major international trial.