Urate oxidase was lost in hominoids during primate evolution. The mechanism and biological reason for this loss remain unknown. In an attempt to address these questions, we analyzed the sequence of urate oxidase genes from four species of hominoids: human (Homo sapiens), chimpanzee (Pan troglodytes), orangutan (Pongo pygmaeus), and gibbon (Hylobates). Two nonsense mutations at codon positions 33 and 187 and an aberrant splice site were found in the human gene. These three deleterious mutations were also identified in the chimpanzee. The nonsense mutation at codon 33 was observed in the orangutan urate oxidase gene. None of the three mutations was present in the gibbon; in contrast, a 13-bp deletion was identified that disrupted the gibbon urate oxidase reading frame. These results suggest that the loss of urate oxidase during the evolution of hominoids could be caused by two independent events after the divergence of the gibbon lineage; the nonsense mutation at codon position 33 resulted in the loss of urate oxidase activity in the human, chimpanzee, and orangutan, whereas the 13-bp deletion was responsible for the urate oxidase deficiency in the gibbon. Because the disruption of a functional gene by independent events in two different evolutionary lineages is unlikely to occur on a chance basis, our data favor the hypothesis that the loss of urate oxidase may have evolutionary advantages.