A 54-year-old woman developed an asymptomatic solitary lung mass, which recurred 10 years after resection. Both lesions consisted of a localized collection of intra-alveolar and interstitial macrophages filled with numerous eosinophilic cytoplasmic crystals. The crystals were non-birefringent, periodic acid-Schiff-negative, and showed polytypic reaction with both kappa and lambda light chains, and alpha, mu, and gamma heavy chains. No immunoglobulin gene rearrangements were detected. This lesion is consistent with crystal-storing histiocytosis, which has been reported in association with plasmacytoma. However, in this case, the absence of monotypic immunoglobulin staining within the crystals or the surrounding plasma cells, the absence of DNA rearrangements, and the long asymptomatic course raise the possibility that this lesion may be reactive. Similar lesions are seen in the lungs of mice with immune defects. Crystal-storing histiocytosis may represent a general reaction pattern to excess immunoglobulin.