We report on pulmonary lesions seen in five cases of idiopathic plasmacytic lymphadenopathy with polyclonal hyperimmunoglobulinemia (IPL). This group of five patients consisted of two Japanese men (age: 33 and 45 years), and three Japanese women (age: 25, 43, and 48 years). All five cases were detected incidentally on routine chest X-rays, and had multiple small nodular lesions in the bilateral lungs. These pulmonary lesions were the initial clinical presentation of IPL in three cases in which, at the onset of disease, no lymphadenopathy was detected. At the disease onset, all five cases showed prominent IPL. In three cases examined, serum interleukin-6 was elevated, and anti-human immunodeficiency type-1 antibody was negative in three cases. Clinically, autoimmune disease was suspected for all five cases, and the various autoantibodies were investigated. Although anti-Scl 70 antibody was positive in one case, this patient had no symptoms of systemic sclerosis. Pathologically, all five lesions were characterized by well-demarcated masses that consisted of abundant reactive germinal centers and a dense lymphoplasmacytic infiltrate in the interfollicular area with a variable degree of interfollicular fibrosis. The immunohistochemical study and polymerase chain reaction demonstrated the polytypic nature of the plasma cells and B-cells. IPL is rare in lymphoproliferative disorders. However, pulmonary involvement may frequently occur in IPL patients. Moreover, pulmonary involvement seems to represent the initial clinical manifestation of IPL. Therapeutically, it is important to discriminate between pulmonary involvement of IPL and pulmonary benign or malignant pulmonary lymphoplasmacytic proliferation, particularly marginal zone B-cell lymphoma of the mucosa-associated lymphoid tissue type.