A series of 222 cases of 7 mm penetrating corneal grafts were analysed with respect to the influence of HLA compatibility. The degree of compatibility was random as no matching was done (HLA types unknown at time of operation). Consequently, most of the cases showed 3 or 4 incompatibilities. The series was divided into seven diagnostic groups (keratoconus, herpetic keratitis, non-herpetic keratitis, stromal dystrophy, endothelial dystrophy, mechanical lesion and corrosion). In all groups there was a tendency towards better results among the compatible transplantation, but only when considering the entire series could statistical significance be demonstrated. The groups of 0--2 incompatibilities showed fewer rejection episodes or opaque grafts than the groups of 3 or 4 incompatibilities (chi 2 = 9.20, P less than 0.005). Comparing only the frequency of opaque grafts among the two groups the correlation was less significant (chi 2 = 3.66, P similar to 0.05).