We examined, immunohistochemically, tissues from primary lung adenocarcinomas. In 142 tumors, the mean overall labeling percentage of cathepsin B was 26.5 +/- 22.3 (SD). The mean labeling percentage of cathepsin B in cases with stage I disease was lower than that in cases with stages IIIA, IIIB, or IV disease (P < 0.05). Of the 115 tumors examined for laminin-positive basement membranes, 54 (47%) had a continuous pattern and 61 (53%) had a discontinuous pattern. The mean labeling percentage of cathepsin B was 35.0 +/- 24.2 in tumors with a discontinuous pattern, compared with the 21.9 +/- 16.9 in those with a continuous pattern (P < 0.01). The overall 5-year survival rates of patients with high and low cathepsin B expressions were 26% and 77%, respectively (P < 0.01), including 45% and 94% for patients with stage I disease, respectively (P < 0.01), and 15% and 60% for those with stage IIIB disease, respectively (P < 0.05). Multivariate analysis using the Cox life table regression model showed cathepsin B to be a significantly independent factor associated with death due to the disease. We conclude from this study that tumors with a discontinuous pattern of laminin have a higher percentage of cathepsin B, and the survival rate was poor for patients with a high expression of cathepsin B. Thus, cathepsin B may be useful in assessing prognosis in lung adenocarcinoma.