Purpose. On the example of a clinical case, to show the complexity of the differential diagnosis of mycobacteriosis of the lungs, mycobacteriosis of the lungs, manifested by a solitary formation, similar to the manifestations of tuberculosis infection and peripheral lung cancer. Materials and methods. Clinical observation is given. A comprehensive examination of the patient, combined with etiological verification methods, in a hospital Saint-Petersburg State Research Institute of Phthisiopulmonology, allowed to make the final diagnosis correctly. Results. Presented clinical observation allowed to demonstrate the difficulties of differential diagnosis of the focal form of mycobacteriosis of the lung with tuberculosis and peripheral lung cancer, and also showed the need for additional diagnostic measures in order to reliably verify the diagnosis in this particular clinical case. Conclusion. The focal form of mycobacteriosis of the lungs is extremely difficult to differentiate with the tuberculosis process in the lungs and peripheral lung cancer, in such cases, a comprehensive examination of patients is required in combination with morphological verification methods and a comprehensive microbiological examination, which allows you to establish the correct diagnosis.