We used complete sequence data from 30 complete Herpesviridae genomes to investigate phylogenetic relationships and patterns of genome evolution. The approach was to identify orthologous gene clusters among taxa and to generate a genomic matrix of gene content. We identified 17 genes with homologs in all 30 taxa and concatenated a subset of 10 of these genes for phylogenetic inference. We also constructed phylogenetic trees on the basis of gene content data. The amino acid and gene content phylogenies were largely concordant, but the amino acid data had much higher internal support. We mapped gene gain events onto the phylogenetic tree by assuming that genes were gained only once during the evolution of herpesviruses. Thirty genes were inferred to be present in the ancestor of all herpesvirus, a number smaller than previously hypothesized. Few genes of recent origin within herpesviruses could be identified as originating from transfer between virus and vertebrate hosts. Inferred rates of gene gain were heterogeneous, with both taxonomic and temporal biases. Nonetheless, the average rate of gene gain was approximately 3.5 x 10(-7) genes gained per year, which is an order of magnitude higher than the nucleotide mutation rate for these large DNA viruses.