Cancer patients frequently develop autoantibodies. To test the hypothesis that the appearance of autoantibodies precedes the clinical diagnosis of cancer, we applied an immunoproteomic approach to compare autoantibody profiles before and after appearance of malignances. Proteins from A549 cells, a lung adenocarcinoma cell line, were separated by two dimensional electrophoresis and then immunoblotted with serum samples from 8 individuals who were eventually diagnosed with lung cancer. Compared with autoantibody profiles from 3 years prior to the appearance of malignances, 21 immunoreactive spots newly appeared or presented with stronger staining intensity when clinical diagnoses were made. Among them, 10 matched spots on 2-DE gels were identified by mass spectrometry analysis as 5 proteins. With immunoprecipitation analysis, the antigenicity of protein cathepsin D was confirmed, and notably, in lung cancer sera, the occurrences of autoantibodies against the specific forms of cathepsin D differed significantly from the control groups (p < 0.05). Our findings suggest that harnessing immunity may have utility for early cancer marker discovery, and that comparing autoantibodies to specific forms of cathepsin D may be a promising early marker of lung cancer. (C) 2012 Published by Elsevier Inc.