BACKGROUND: Lung cancer (LC) is the most common fatal malignancy, but there are no useful tumor markers for diagnosis or monitoring. Mucin 1 has an established role as a marker in other malignancies, but has undergone limited assessment in LC. METHODS: Serum from 86 patients with LC and 24 with benign pulmonary disease (BPD), and bronchial lavage fluid from 55 LC patients and 21 BPD patients were tested using the Mucin 1 assays mammary serum antigen (MSA) and cancer-associated serum antigen (CASA). RESULTS: For LC, serum CASA achieved sensitivity of 57%, specificity of 93% relative to normals, and 63% specificity relative to BPD. For MSA the same parameters were 19%, 95%, and 92%. Serum CASA levels were significantly higher in LC patients compared with BPD (P = 0.024) but there was no difference for MSA (P = 0.635). CASA showed excellent correlation with tumor stage and in patients with changing status of disease, while MSA did not. By contrast there was no difference in bronchial lavage fluid tumor marker levels from LC and BPD patients (CASA, P = 0.87; MSA, P = 0.89). CONCLUSIONS: In a small series serum CASA appears to be a useful agent in detecting LC because it is elevated in all types and stages of LC, and its level correlates with stage and progress of disease. Some patients with BPD have elevated levels suggesting a greater value for monitoring rather than diagnosis. Both serum MSA testing and measurements of either marker in bronchial lavage fluid are of no value.