The Collaborative Ocular Melanoma Study (COMS) is the largest multicenter clinical trial ever sponsored by the National Eye Institute. During the past year, COMS clinic monitoring procedures disclosed some apparent misreporting of data at two of the 43 participating clinical centers. With full cooperation and support from the respective principal investigators, an immediate audit was conducted at these study centers (Northwestern University in Chicago, Ill, and the Cleveland Clinic Foundation in Ohio) to ascertain the extent to which instances of misreporting may affect the study outcomes. On October 4 and 5, 1993, the results of these initial audits were presented to the COMS Data and Safety Monitoring Committee and Executive Committee and to the National Eye Institute leadership. Based on information available at that time, all three groups judged that it was unlikely that the deviations would affect the interpretation of COMS data with respect to the primary study outcomes,