A 48-year-old man was admitted for evaluation of abnormal shadows on chest radiograph. Chest computed tomography (CT) showed cysts, nodules, and cervical and axillary lymphadenopathies. Elevated serum levels of IgG4 and interleukin (IL)-6 suggested IgG4-related disease (IgG4-RD) or multicentric Castleman's disease (MCD). Histologic findings of the cervical lymph node and right lung S6 biopsies revealed numerous IgG4-positive plasma cells. Although CT findings of the lungs were atypical for IgG4-RD, consistent histologic findings, clinical symptoms, and laboratory data made us conclude IgG4-RD. Because histologic findings of IgG4-RD and MCD have similarities, differentiating between the two diseases should consider the clinical presentation.