Processes involved in malignant transformation of the lung from preneoplasia are poorly understood. To better understand this process, two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) profiles of proteins from the normal, metaplasia, dysplasia and carcinoma tissues of human bronchial epithelia were examined by differential proteomic analysis. The selected differential protein-spots were identified by peptide mass fingerprint based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and database searching. The average spots for normal epithelium, metaplasia, dysplasia and invasive carcinoma were 1189.50 +/- 39.89, 1227.00 +/- 37.90, 1273.00 +/- 43.31 and 1326.00 +/- 66.63, respectively. Well-resolved, reproducible 2-D PAGE patterns of the normal-metaplasia-dysplasia-carcinoma tissues of bronchial epithelia were obtained. After matching, the number of spots of differential proteins between normal tissue and metaplasia, metaplasia and dysplasia, and dysplasia and invasive cancer tissues were 31.50 +/- 7.67, 41.00 +/- 9.07 and 56.00 +/- 8.96, respectively. In total, 35 differential proteins, expressed only at the later stage of a two-stage comparison, were identified, some of which are known to be involved in regulating the processes of proliferation, differentiation and signal transduction. Current data in this study, for the first time, provide the basis for identification of potential tumor markers of human lung squamous carcinoma and their involvement in the progression of malignant transformation of bronchial epithelium.