We describe herein how true lymphoma of the lung was differentiated from pseudolymphoma in a 45-year-old woman presenting with pulmonary infiltrates. Although segmental resection revealed typical histologic findings of pseudolymphoma of the lung and immunohistochemical studies did not demonstrate a monoclonal proliferation, Southern blot analysis of the frozen tissue revealed rearrangements in the heavy and light chains of the immunoglobulin gene, with no T-cell receptor gene rearrangement suggestive of a lymphoproliferative disorder. These findings indicate that the identification of gene rearrangement may be utilized to distinguish between true lymphoma and pseudolymphoma.