Tumor Necrosis Factor (TNF) was determined in the serum of 72 lung carcinoma patients. Twenty-four healthy subjects younger than 50 years and 10 healthy subjects older than 70 years were considered as control group. TNF was also measured in 20 patients with stage I sarcoidosis and in 15 patients with pulmonary fibrosis. The marker was detected in 32% of cases in the neoplastic group, in 37.5% of disease confined to the chest and in 25% of advanced disease cases. A large proportion of TNF-positive samples was found in sarcoidosis (30%), and even larger in pulmonary fibrosis (66.6%). TNF was also present in healthy subjects older than 70 (40%). We conclude that TNF is not specific of malignancy, being demonstrable in other benign pulmonary diseases and even in the course of physiological aging.