Background: To investigate graft survival after full--thickness corneal xenotransplantation from alpha 1,3-galactosyltransferase gene-knockout (GTKO) pigs expressing a human complement regulatory protein (GTKO/CD46 pigs) in rhesus monkeys. Methods: Rhesus monkeys (n= 10) were transplanted with full--thickness corneas from wild-type (WT; n= 4) and GTKO/CD46 (n= 4) pigs or from monkeys (n= 2). All recipient monkeys received post-transplant subconjunctival injections of betamethasone. Corneal grafts were evaluated by slit-lamp. Histopathology, immunohistochemistry of the grafts, and cytokine concentrations in the aqueous humor were tested 6 months after transplantation. Anti-pig IgM/IgG and anti-galactose-alpha 1,3-galactose (Gal) antibodies were determined by flow cytometry and ELISA, respectively. Results: The longest graft survival of WT and GTKO/CD46 xenografts was 157 and 171 days, respectively. There was no significant difference in graft survival between WT and GTKO pig corneas. Anterior synechiae occurred in two recipients of WT and all recipients of GTKO/CD46 grafts. All xenograft recipients developed a retrocorneal membrane, inflammatory cells (CD3+ T lymphocytes) infiltrated the corneal stroma, and in the aqueous humor, IL-6 was increased 6 months after transplantation. Induced antibody responses were documented against Gal and/or non-Gal pig antigens. In contrast, allografts survived > 180 days without any rejection, with no increase in cytokines in the aqueous humor, and no elicited serum anti-pig antibodies were detected. Conclusions:alpha 1,3-galactosyltransferase gene-knockout/CD46 pig corneas were not associated with prolonged graft survival or a reduced antibody response compared with WT pig corneas. The prevention of the development of anterior synechiae and a retrocorneal membrane after corneal xenotransplantation would appear to be important if prolonged corneal xenograft survival is to be achieved.