OBJECTIVE To investigate the cause of an acute respiratory tract infection (ARTI) outbreak. METHODS Thirty-eight clinical samples were collected from 19 patients in an ARTI outbreak that occurred in a physical training facility in January 2013; patient demographic information was also collected. In addition, 60 influenza virus-negative samples from febrile respiratory patients were collected from the same community at the same time to determine whether these were the same infections. Multiplex PCR (multi-PCR) was used to detect the possible pathogen in these samples. All human adenovirus (HAdV)-positive samples were inoculated onto Hep-2 cells for isolation. HAdV isolates were typed by hexon gene, fiber gene, and whole genome sequencing using primers designed in-house and compared to different type/serotype HAdVs downloaded from GenBank. Phylogenetic analysis was used to determine the type of the HAdV. RESULTS Of the 38 samples, 34 from 17 cases were HAdV-positive; two of them were co-infected, one with respiratory syncytial virus A and the other with human rhinovirus. The hexon gene open reading frame (ORF; 2841 nucleotides (nt)) and fiber gene ORF (978 nt) were obtained from four HAdV strains (TJ-2013-92, TJ-2013-94, TJ-2013-100, TJ-2013-122) from three upper respiratory infection cases and one pneumonia case. They were all completely identical. One HAdV isolate, TJ-2013-90, was selected for whole genome sequencing; 34238 nt were obtained. Phylogenetic analysis showed the whole genome of TJ-2013-90 to be clustered together with HAdV-B55/HAdV-B11a. Three of 60 influenza virus-negative specimens were HAdV-positive, but hexon and fiber gene analysis showed that they were grouped in different branches to the HAdV isolates from this outbreak. CONCLUSIONS The cause of this ARTI outbreak was HAdV-B55. This was another outbreak caused by this re-emerging virus. Continuous surveillance of respiratory adenovirus is necessary for disease control.