Objective To analyze the clinical characteristics and peripheral blood lymphocyte subsets of HAdV55 pneumonia in adult patients. Methods 132 adult patients with HAdV55 infection in a hospital from December 2016 to May 2018 were retrospectively analyzed, including 38 patients with pneumonia (pneumonia group) and 94 patients with upper respiratory tract infection (URTI group), clinical and imaging characteristics of two groups of patients were compared. Flow cytometry was used to analyze the distribution of peripheral blood lymphocyte subsets in 31 patients with pneumonia, 38 patients with URTI and 10 healthy controls, dynamic changes in peripheral blood T lymphocyte subsets in 5 patients with pneumonia and 3 patients with URTI were observed. Results Compared with URTI group, patients in pneumonia group had higher proportion of expectoration, longer fever days and hospitalization time, decreased peripheral blood white blood cell and neutrophil count, as well as increased lymphocyte proportion and absolute count (all P < 0.05), chest CT showed that the lung lesions mainly focused on the right lung (21/38, 5.3%). Lymphocyte subsets analysis showed that proportions of CD4⁺T cells, CD8⁺T cells, NK cells and B cells in patients with pneumonia were both higher than those in healthy controls, there was no significant difference compared with patients in URTI group (P>0.05). The dynamic analysis of T lymphocyte subsets showed that CD3⁺ T lymphocyte count of 5 pneumonia patients decreased on the 3rd day of disease course, then increased gradually, and returned to normal on the 18th day. Conclusion Expectoration is more common in patients with HAdV55 adult pneumonia, right lung is easy to be affected, host cell immune function is disordered, dynamic monitoring on peripheral blood lymphocyte subsets is helpful to assess the disease condition.