The Collaborative Corneal Transplantation Studies are a pair of multicenter prospective clinical trials evaluating the effectiveness of histocompatibility matching in high risk keratoplasty patients. The antigen matching study (AMS) evaluated HLA matching in patients without circulating lymphocytotoxic antibody to HLA antigens and the cross-match study (CS) evaluated the effect of using cross-match-negative donors in patients with identified circulating lymphocytotoxic antibodies to HLA antigens. Sera from 510 patients considered for enrollment in the studies were screened preoperatively for the presence of anti-class I lymphocytotoxic antibodies (LA). The 42 patients (8%) found to have detectable LA entered the CS. The 468 patients found not to have detectable LA preoperatively entered the AMS. Fifteen of the 37 transplanted CS patients were found to have donor-specific anticlass I antibody (before or after surgery). These patients were also screened for anti-class II LA and 25 had anti-class II panel reactive antibody ≥ 5%.Forty-nine of the 419 transplanted AMS patients (12%) were found to have produced anti-class I LA after surgery, and in 19 patients, antibody specificities were those of donor HLA antigens. There was a significant association between the number of mismatched class I antigens and the number of donor-specific LA produced. The production of LA by AMS patients was significantly associated with reaction episodes; eighty-two percent of patients (40 of 49) with LA had reaction, compared with 63% of patients (230 of 365) without LA (P=0.02). Likewise, production of donorspecific LA was significantly associated with immunemediated graft failure (P=0.025).For CS patients, there was no correlation between the production of donor-specific anti-class I or nonspecific anti-class II antibodies and graft outcome. However, the CS patients had poorer graft survival than did AMS patients at 3 years (57% vs. 66%,P=0.01). These data demonstrate that LA, especially directed against donor class I HLA antigens following corneal transplantation in high risk patients, are associated with immune graft rejection and can be an indicator of allograft rejection.