The mortality of lung cancer remains high, despite improved diagnostic techniques that allow small lung tumors to be detected. In this study, we evaluated the prognostic significance of the tracheal mucin MUC4 by immunohistochemical investigation of the expression profiles of MUC4, ErbB2, p27 and MUC1 in lung adenocarcinoma specimens (non-bronchiolo-alveolar type, < or =3cm) from 185 patients. MUC4 is a membrane mucin, similarly to MUC1, and in addition MUC4 functions as an intra-membrane ligand for receptor tyrosine kinase ErbB2 and is associated with regulation of p27. However, MUC4 expression was found to be unrelated to expression of MUC1, ErbB2 and p27 in small-sized lung adenocarcinomas. The disease-free interval (DFI) and survival rate of 25 patients with high MUC4 expression (> or =25% of neoplastic cells stained) were significantly lower than those of 160 patients with low MUC4 expression (<25% of neoplastic cells stained) (P<0.05), whereas ErbB2 and p27 expression showed no significant correlation with DFI and survival. Univariate analysis showed that high MUC4 and p27 expression correlated with blood vessel invasion (P=0.0004), and MUC4 expression was frequently detected in regions of stromal invasion. In addition, the survival rate of stage IA patients with high MUC4 expression was significantly lower than that of stage IA patients with low MUC4 expression (P<0.05). In conclusion, high MUC4 expression in small-sized lung adenocarcinomas correlates with a short DFI and a poor survival rate. Therefore, MUC4 expression might be a new independent factor for prediction of outcome and indication of poor prognosis in lung adenocarcinoma.