We found that high pleural fluid concentrations of 2 classical tumor markers (i.e., CA15-3 and CA19-9) simultaneously, or alternatively an elevated CA19-9 along with low kallikrein-12 concentrations, pointed to a metastatic lung adenocarcinoma rather than mesothelioma, with 65% sensitivity and 100% specificity. This evidence concerns the gene KLK4 and mesothelioma.