We describe the case of a 50-year-old woman with a lung tumor composed of crystal-storing histiocytes. These cells and associated plasma cells failed to show clonal light chain restriction, and the patient had no associated hematologic disorder. The differential diagnosis included crystal-storing histiocytosis, characterized by accumulation of crystallized immunoglobulins, a rare manifestation of monoclonal gammopathies/plasma cell dyscrasias. Crystal-laden histiocytes have previously been described in many organs. Four reports have described crystal-storing histiocytosis in the lung, always associated with a lymphoproliferative disorder. The present patient, 1 other case from our archive, and 1 case reported in the literature, all without an association with lymphoproliferative disorder, make a full description and definition of this lesion appropriate. The morphology, immunohistochemical profile, and electron microscopic features are described herein, and the term pulmonary crystal-storing histiocytoma is proposed. A practical algorithm is presented for the assessment of solitary lung masses composed of large histiocytic cells.