The Collaborative Corneal Transplantation Studies Group conducted two controlled, double-masked studies addressing distinct scientific questions about donor-recipient histocompatibility matching. The Crossmatch Study was a randomized study assessing the effectiveness of crossmatching in preventing graft rejection among high-risk patients with lymphocytotoxic antibodies. The Antigen Matching Study was a prospective, double-masked, observational study of the effectiveness of HLA-A, -B, and -DR donor-recipient matching in high-risk patients who had no lymphocytotoxic antibodies.