Presence of triiodothyronine's receptors has been confirmed in majority of cells constituting lung architecture. The purpose of the investigation is the estimation of the conversion of the thyroxine through examination the activity of deiodinase I in the lung. The material in the form of 2.0-3.0 g lung pieces was taken in therapeutic thoracotomy from patients with emphysema, and in non-small cell lung cancer (n.s.c.l.c.). The examination of the activity of deiodinase I in chronic pulmonary diseases can help to show one of the elements of euthyroid sick syndrome (e.s.s.). The correlation between the deiodinase I activity in neoplastic tissue and the extent of the process of n.s.c.l.c. may provide the necessary data about the behaviour of triiodothyronine receptors which are closely related to oncogenes. The deiodinase I activity may be considered as a marker of lung cancer.