Immunoglobulin binding protein 1 (IGBP-1) was initially identified as a signal transduction molecule coprecipitating with MB1 (Igα) of the B cell antigen receptor (BCR) complex and was later found to be broadly expressed. Immunoglobulin binding protein 1 has been characterized as an associated and regulatory component of the catalytic subunits of protein phosphatase 2A (PP2A), which is the most abundant phosphatase and plays important roles in cell growth and cell cycle control. The aim of this study was to investigate the expressional characteristics of IGBP-1 and PP2Ac during pulmonary adenocarcinogenesis. The positivity rate of IGBP-1 increased during the course of sequential progression from non-invasive carcinoma (8/30, 26.7%) to invasive adenocarcinoma (37/46, 80.4%) among cases that showed areas of lepidic growth. In contrast, all of the small adenocarcinomas showing a non-lepidic growth pattern were positive for IGBP-1 (20/20, 100%). All cancers that proved ultimately fatal were positive for IGBP-1, and log-rank analysis showed that IGBP-1 positivity was significantly correlated with a poor prognosis. In contrast, atypical adenomatous hyperplasias and lung adenocarcinomas were uniformly positive for PP2Ac. Protein phosphatase 2A was not associated with carcinoma progression. Thus we have demonstrated that IGBP-1 is expressed universally in advanced lung adenocarcinomas, and that its overexpression is significantly related to outcome.