Background: Malignant mesothelioma is an aggressive, uniformly fatal tumor. Serum markers would be useful for the diagnosis of this disease. One potential marker is mesothelin. The purpose of this study was to study the mesothelin biomarker in a large patient cohort and to determine if another biomarker, CA125, improves on the sensitivity of mesothelin in the diagnosis of mesothelioma.Methods: Serum levels of mesothelin and CA125 were determined by commercially available assays in 117 samples obtained at diagnosis from patients with pleural malignant mesothelioma, 33 healthy, asbestos-exposed individuals, 53 patients with asbestos-related lung or pleural disease, and 30 patients presenting with benign pleural effusions. Cross-validated sensitivities were determined, and receiver operator characteristic curves were generated to compare the diagnostic accuracy of the biomarkers.Results: CA125 had a cross-validated sensitivity of 27% for mesothelioma patients at a specificity of 95% relative to asbestos-exposed individuals, or 50% relative to individuals with benign pleural effusions. Mesothelin had a cross-validated sensitivity of 52% for mesothelioma. patients, at a sensitivity of 95% relative to individuals with benign lung or pleural disease. CA125 and mesothelin levels were discordant in > 50% of mesothelioma patients. Combining the data from the two biomarkers using a logistic regression model did not improve sensitivity for detecting mesothelioma above that of the mesothelin marker alone.Conclusion: Combining mesothelin and CA125 data does not improve the sensitivity of mesothelioma. diagnosis over mesothelin alone. The use of both markers potentially increases the number of patients who can be monitored.