The COMS is conducted in 43 clinical centers located in major population areas of the United States and Canada. Six resource centers participate and have major roles in quality assurance for the study. Information gathered and analyzed includes time to death from all causes, time to death from cancer (whether metastatic choroidal melanoma or not), diagnosis of other tumors, complications of radiation, and changes in visual acuity. A parallel study of quality of life for patients enrolled in the trial of radioactive plaque was initiated in January 1995. From November 1986 through July 1998, 8,712 patients with choroidal melanoma of all sizes were screened for eligibility for a COMS clinical trial.