AbstractThe Data and Safety Monitoring Committee (DSMC) is a committee independent of both the Steering Committee (SC) and any sponsor. It advises these bodies on continuation or stopping based upon safety and efficacy considerations. The primary objective is to assure safety for the patients in the trial. Blinding of the DSMC regarding type of treatment is usually avoided. The DSMC is composed of three to five members and should include one or two clinicians knowledgeable in the field of the trial, one or two statisticians and in some situations a pharmacologist and an ethicist. It is important that the members have experience from other trials and have high integrity. Rules for the statistical analyses have to be set up in the beginning of the trial. However, the DSMC cannot rely only upon statistical rules. Information from other sources may cause stopping before the scheduled end of the trial. The DSMC has to check that the overall quality of the data is good. Of special importance is that the Endpoint Committee is current with classification of endpoints. Timing of the DSMC meetings is dependent on several factors: (i) the incidence of events; (ii) the power of the study design; (iii) the effectiveness of the intervention. Legal requirements to report serious adverse events to legal authorities may be taken over by the DSMC, which ideally is the only body that has access to unblinded data. Copyright © 2002 John Wiley & Sons, Ltd.