OBJECTIVE To describe the rationale for independent data monitoring committees (DMCs) for National Cancer Institute (NCI)-sponsored phase III cooperative group clinical trials. METHODS We review the necessity for interim monitoring of outcome data during the course of randomized clinical trials and summarize the reasons for establishing DMCs with requisite expertise and with appropriate independence from study investigators. RESULTS The important components of the policy for cooperative group DMCs are described with a focus on the makeup of these bodies and on the complementary roles of study committee leadership and DMCs in protecting patient safety during the conduct of randomized clinical trials. CONCLUSIONS The cooperative group DMCs that are independent of the study committees and that have the requisite expertise to examine accumulating data and to base decisions on monitoring guidelines that are specified in advance by the study committee provide a body able to protect patient safety, to protect the integrity of the clinical experiments on which patients have consented to participate, and to assure the public that conflicts of interest do not compromise either patient safety or trial integrity.