OBJECTIVE: To describe the composition and role of the data safety and monitoring board (DSMB) for the National Institute of Mental Health (NIMH) Collaborative HIV/STD Prevention Trial. DESIGN: NIMH appointed to the DSMB nine members representing the following areas of expertise: prevention science, ethnography, infectious diseases (especially HIV and sexually transmitted diseases), laboratory diagnostics, clinical practice, methodology, international trial experience, statistics, and ethics. METHODS: The DSMB assessed the overall study for any concern about plans or implementation and reviewed cumulative study data to evaluate the safety of study participants, the ongoing conduct of the study, and the scientific validity and integrity of the Trial. Because of the Trial's international scope, the DSMB examined the effects of cultural differences on study implementation and fidelity. RESULTS: Among the DSMB recommendations that strengthened the Trial was one to conduct initial epidemiological studies of the venues selected for the intervention to verify risk and to establish intraclass correlation coefficients that could be used to calculate appropriate sample sizes. CONCLUSIONS: The DSMB played a critical role in this Trial. Because members have the expertise required to monitor the Trial, are not involved in the daily management of the Trial, and can review interim analyses and adverse event reports, they are in an excellent position to provide expert advice to ensure that the Trial's goals are achieved and that NIH funds are well invested. © 2007 Lippincott Williams & Wilkins, Inc.