The effects of selenium, an essential nutrient with anti-carcinogenic properties, are mediated by selenium-binding proteins. The protein expression status of human selenium-binding protein 1 (SBP1) in human tumours and the exact function of this protein are not known. In this study, quantitative two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) was used on 93 lung adenocarcinomas and ten uninvolved lung samples. Two likely isoforms of a 56 kD protein that showed a significantly decreased abundance in lung adenocarcinomas were observed. Tandem mass spectrometry and 2-D western blot analysis identified these two proteins as human SBP1. Tumour tissue microarrays were utilized to examine the cellular expression patterns of SBP1 using immunohistochemistry. The same tissue samples were examined for SBP1 mRNA expression using oligonucleotide microarrays. Two major SBP1 isoforms were detected, with an acidic isoform (457) being significantly down-regulated in lung adenocarcinomas compared with normal lung (p = 0.02). Two additional more acidic SBP1 isoforms were only observed in normal lung. SBP1 protein isoforms and SBP1 mRNA levels were significantly decreased in poorly differentiated (versus moderately and well-differentiated), T2-T4 (versus T1), and bronchus-derived (versus bronchioloalveolar) tumours. Low levels of SBP1 protein (native form, 460) correlated significantly with poor survival (p = 0.007). The lack of SBP1 expression was not due to gene deletion. Treatment of A549 lung adenocarcinoma cells with the methylation inhibitor 5-azacytidine did not affect expression of the SBP1 protein. Analysis of the tumour proliferation status using Ki-67 suggests that down-regulated expression of SBP1 may reflect increased cell proliferation and decreased differentiation in lung adenocarcinomas.