In the study of 65 matched pairs of lung tumor tissue and normal lung parenchyma, cathepsin B (CB) activity was found to be increased about 4.6-fold, when regarding median levels. CB activity was found to be insignificantly higher in adenocarcinoma compared to the other histologic cell types. CB levels did not correlate with tumor stages (TNM) nor with cell differentiation, while increased CB activity was related to shorter survival rates of the patients. It is concluded that lung tumor CB is of some prognostic value for the outcome of the malignant disease.