Seven hexon hypervariable regions (HVRs) of adenoviruses (Ads) were identified by comparing the regions among different serotypes; however, no one has compared HVR sequences among the identical serotypes, except for adenovirus type 3 (Ad3). To examine a variability between the HVRs for each serotype, we compared the sequences of Ad1-6 isolates, respectively, isolated between 1988 and 2007 in Yamagata, Japan. We selected 23-43 isolates randomly and sequenced 894-987 bp regions. Except for strains with insertions and deletions, the sequence identities among Ad1-6 were 99-100%, excluding that between the two Ad5 groups (approx. 94%). Even the insertions and deletions were likely to be established, as these changes were repeatedly observed. The obtained phylogenetic tree indicated that Ad isolates and reference strains branched depending on serotype. The Yamagata isolates had similar sequences or amino acid arrangements to the reference strains as well as to other strains isolated in different areas. HVRs have been stably conserved as serotype-specific regions for a long period with only minor genomic variations. Therefore, we herein recommend that these regions be hereafter referred to as "serotype-specific regions", which might be a more appropriate title with which to characterize the epidemiological nature of these sites than the current "HVRs".