MHC class I polymorphism improves the defense of vertebrate species against viruses and other intracellular pathogens. To see how polymorphism at the same class I genes can evolve in different species we compared the MHC-A, MHC-B, and MHC-C loci of common chimpanzees and humans. Diversity in 23 Patr-A, 32 Patr-B, and 18 Patr-C alleles obtained from study of 48 chimpanzees was compared to diversity in 66 HLA-A, 149 HLA-B, and 41 HLA-C alleles obtained from a study of over 1 million humans. At each locus, alleles group hierarchically into families and then lineages. No alleles or families are shared by the two species, commonality being seen only at the lineage level. The overall nucleotide sequence diversity of MHC class I is estimated to be greater for modern chimpanzees than humans. Considering the numbers of lineages, families, and alleles, Patr-B and Patr-C have greater diversity than the HLA-B and HLA-C, respectively. In contrast, Patr-A has less polymorphism than HLA-A, due to the absence of A2 lineage alleles. The results are consistent with ancestral humans having passed through a narrower population bottleneck than chimpanzees, and with pathogen-mediated selection having favored either preservation of A2 lineage alleles on the human line and/or their extinction on the chimpanzee line.