Multicentric Castleman's disease (MCD) is an uncommon and often incurable lymphoproliferative disorder. There has been some recent evidence that rare cases of MCD manifest diffuse lung involvement, but the features in these cases are not well characterized. We report just such a biopsy-proven case of MCD with typical laboratory abnormalities including serum interleukin-6 elevation and characteristic high-resolution CT findings. Immunopathologically, the features of the lung tissue resembled those of lymphocytic interstitial pneumonia with predominant infiltration of B cells and plasma cells. In addition, the abnormal appearance of B cells in bronchoalveolar lavage fluid was of diagnostic value. Although MCD is often refractory to treatment including corticosteroid, chemo- and immuno-therapy, we show successful treatment with corticosteroid and cyclophosphamide and 4 years of complete remission.