The innate immune system includes antimicrobial peptides that protect multicellular organisms from a diverse spectrum of microorganisms. beta-Defensins comprise one important family of mammalian antimicrobial peptides. The annotation of the human genome fails to reveal the expected diversity, and a recent query of the draft sequence with the blast search engine found only one new beta-defensin gene (DEFB3). To define better the beta-defensin gene family, we adopted a genomics approach that uses hmmer, a computational search tool based on hidden Markov models, in combination with blast. This strategy identified 28 new human and 43 new mouse beta-defensin genes in five syntenic chromosomal regions. Within each syntenic cluster, the gene sequences and organization were similar, suggesting each cluster pair arose from a common ancestor and was retained because of conserved functions. Preliminary analysis indicates that at least 26 of the predicted genes are transcribed. These results demonstrate the value of a genomewide search strategy to identify genes with conserved structural motifs. Discovery of these genes represents a new starting point for exploring the role of beta-defensins in innate immunity.