Immunoglobulin G4-Related Disease (IgG4-RD) is a systemic fibro-inflammatory disease that has been proposed as a separate entity since the beginning of this century. The disease is often manifested by increased serum IgG4 levels and certain histopathological manifestations. The patient mentioned in this article is a 29-year-old man from Tajikistan, who has had a chronic cough since the beginning of 2018 without a previous history of the disease. At first, he was diagnosed with pneumonia for a long time and then underwent a lung biopsy due to exacerbation of symptoms and the spread of lung lesions in radiology but no abnormalities were found in these evaluations. The patient traveled to Iran to continue his treatment. He was re-evaluated and then the previous samples taken from the patient's lung tissue were re-examined. There were key findings in favor of diagnosing IgG4 RD. Evaluations did not confirm the involvement of other organs. He was first treated with steroids and due to recurrence of symptoms, he was treated with rituximab once which was significantly effective in improving the patient's clinical symptoms. In general, it can be concluded that the diagnosis of IgG4-RD is very challenging and if it has not been diagnosed and treated in time, it can lead to irreversible fibrosis and permanent loss of function of the involved organ.