Immunoglobulin G4-Related Disease (IgG4-RD) is a systemic fibro-inflammatory disease that has been proposed as a separate entity since thebeginning of this century. The disease is often manifested by increasedserum IgG4 levels and certain histopathological manifestations. The patientmentioned in this article is a 29-year-old man from Tajikistan, who has had achronic cough since the beginning of 2018 without a previous history of thedisease. At first, he was diagnosed with pneumonia for a long time and thenunderwent a lung biopsy due to exacerbation of symptoms and the spreadof lung lesions in radiology but no abnormalities were found in theseevaluations. The patient traveled to Iran to continue his treatment. He wasre-evaluated and then the previous samples taken from the patient's lungtissue were re-examined. There were key findings in favor of diagnosingIgG4 RD. Evaluations did not confirm the involvement of other organs. Hewas first treated with steroids and due to recurrence of symptoms, he wastreated with rituximab once which was significantly effective in improving thepatient's clinical symptoms. In general, it can be concluded that thediagnosis of IgG4-RD is very challenging and if it has not been diagnosedand treated in time, it can lead to irreversible fibrosis and permanent loss offunction of the involved organ.