We describe a patient with Waldenstrom macroglobulinemia who presented with a lymphocytic pleural effusion. Pleural involvement in Waldenstrom macroglobulinemia is very rare. In addition, to our knowledge, there are no reports in the literature in which a primary diagnosis was made on the basis of pleural fluid analysis. The presence of small lymphocytes in serous cavity fluid can pose great difficulty in the differentiation between a low-grade lymphoproliferative disorder and reactive lymphocytosis. The diagnosis of malignancy in this case was established on the basis of morphologic testing, flow cytometry, and the detection of B-cell immunoglobulin gene rearrangement.