Adenovirus type 7h is currently the predominant virulent genome type of serotype 7 isolated in Argentina, Chile, and Uruguay in association with severe infantile pneumonia. In order to characterize possible molecular determinants of pathogenicity, the nucleotide sequence of a 5904-bp fragment (76 to 93 mu) containing the entire E3 region and the fiber gene of Ad7h was established. The organization of the ORFs within the E3 region was similar to that reported for the prototype strains of Ad7 and Ad3. A comparison of the nucleotide and amino acid sequences of all ORFs revealed a higher homology between Ad7h and Ad7p than between Ad7h and Ad3 for 12.0K and 16.1K, whereas the 15.3K ORF and the adjacent fiber gene were strikingly more homologous to those of Ad3 (99.5 vs 81.1% and 98.2 vs 66.6%, respectively). The equivalent to ORF 7.7K in Ad7p was missing in Ad7h due to a deletion and a mutation affecting the start codon (ATG-->ATT). Although the hemagglutinin of the Ad7h fiber could not be characterized due to its lack of activity on monkey erythrocytes, our results indicate that Ad7h is an intermediate strain 7-3.