The National Cancer Institute funds and coordinates a large international system of clinical trials, involving many large and small institutions and thousands of patients, which is organized into a dozen cooperative groups. Compliance with the study requirements and data accuracy are monitored at several levels within these collaborative groups. In addition, for approximately 9 years an on-site data audit process has been conducted by each group. We report the organization and audit methods of one of these groups, the CALGB. In 3 cycles of auditing each CALGB institution, involving over 2300 patients, there have been only 2 instances of scientific improprieties, a rate of only 0.4% in 459 institutional audits. A more worthy result of this on-site data auditing has been the improvement in compliance with protocol and administrative requirements. Although a system of on-site reviews of patient records expends both scarce research funds and investigator time, the CALGB has demonstrated the value of such audits for improving the quality of clinical research. The audit system developed by CALGB could be used as a model for others to verify clinical research.