A 54-year-old male was found to have an enlarged right inguinal lymph node and an abnormal chest shadow. Although a lymph node biopsy was performed, no definite diagnosis was made. Medical check-ups 4 years later detected hypergammaglobulinemia, elevated serum C-reactive protein levels, enlarged portal lymph nodes and splenomegaly. Chest radiography showed reticular shadows in bilateral lower lung fields and consequently the man was referred to our hospital. Lung specimens taken by video-assisted thoracoscopic lung biopsy revealed plasmacytic infiltration along the lymphatic routes. HHV8 and IgG4 were both negative in the specimens, and no malignant findings were observed. The previously biopsied right enlarged inguinal lymph node also revealed plasmacytic infiltration. Based on these findings, a diagnosis of multicentric Castleman disease (MCD) was confirmed. As no pulmonary function impairment or severe symptoms were observed, it was considered a mild case of MCD. The patient remains under observation and is at present not receiving active treatment.