Lung fibrosis is an important pulmonary disease with a high mortality rate, but its pathophysiological mechanism has not been fully clarified. Various types of cells have been implicated in the development of lung fibrosis, including T cells. However, the contribution of functional molecules expressed on T cells to the development of lung fibrosis remains largely unknown. In this study, we determined whether costimulation via CD28 on T cells was crucial for the development of lung fibrosis by intratracheally administering bleomycin into CD28-deficient mice. Compared with wild-type mice, the CD28-deficient mice showed markedly impaired lung fibrosis after injection with low doses of bleomycin, as judged by histological changes and hydroxyproline content in the lungs. In addition, bleomycin-induced T cell infiltration into the airways and production of several cytokines and chemokines including IL-5 were also impaired in the CD28-deficient mice. Furthermore, adoptive transfer of CD28-positive T cells from wild-type mice recovered the impaired bleomycin-induced lung fibrosis in CD28-deficient mice. These findings suggest that the CD28-mediated T cell costimulation plays a critical role in the development of lung fibrosis, possibly by regulating the production of cytokines and chemokines in the lung. Thus, manipulation of the CD28-mediated costimulation could be a potential therapeutic strategy for the prevention of lung fibrosis.