In nine rhesus monkeys, human corneal grafts were used for penetrating keratoplasty in one eye of each. The data were divided into three groups according to the size of grafts. Group 1 (n=3) received 5.5-mm human corneal grafts in 5.0-mm recipient beds; group 2 (n=3) received 6.2-mm human corneal grafts in 6.0-mm beds; and group 3 (n=3) received 7.5-mm grafts in 7.0-mm beds. Five of nine eyes in groups 1 and 2 (55%) maintained graft clarity for more than 1 year, but only two of the three in group 1 maintained clear human corneal grafts for 32 and 36 months, respectively. However, the three grafts in the third group maintained transparency for only 4-7 months. Our findings suggest that xenogeneic corneal grafts may have a greater success rate in humans than previously believed.