The origin of human adenovirus type 4 (Ad4), an important pathogen and candidate vaccine vector, has been the subject of speculation. Ad4 is unusual among adenoviruses, because it is the single known serotype of subgroup E. Some biological and biochemical properties of Ad4 resemble those of serotypes from subgroups B and C. The length of Ad4 fiber is intermediate between that of subgroup B and C fibers. We sequenced the Ad4 fiber gene, locus of the determinant(s) of adenovirus serotype. The number of repeating DNA sequence motifs in the shaft domain of the Ad4 fiber gene is consistent with its reported length. Regional phylogenetic analysis of Ad4 was undertaken, comparing DNA sequences of early genes and fiber genes from representative adenoviruses. The Ad4 fiber gene has close phylogenetic relationship to subgroup C fiber genes. This is in distinct contrast to the closer relationship of Ad4 to subgroup B adenoviruses in early gene sequences, distributed across the left 70% of the viral genome. We propose that Ad4 originated by recombination of genomes resembling contemporary subgroup B and subgroup C adenoviruses. This event may have occurred so recently that divergence of subgroup E serological determinants has yet to be observed.