The paper describes a case of computed tomography (CT)-guided transthoracic biopsy (TTB) of a neoplasm of the single functioning lung in a 69-year-old patient with polyneoplasm of the lung and prostate. During the current treatment for lung cancer, the patient noted the progression of the process as an appearance and increase of foci in the right lung. After a previous left upper lobectomy and subsequent external beam radiation therapy, the patient developed fibroatelectasis of the remaining left lower lobe. The purpose of the biopsy is to identify the source of metastasis, to conduct a molecular genetic study of the tumor, and to choose therapy. The patient underwent CT-guided TTB using stepwise and coaxial procedures. Histological and immunohistochemical studies confirmed the presence of lung adenocarcinoma metastasis. A molecular genetic study revealed EGRF gene exon 19 and 20 mutations, which made it possible to change the therapy.