Background: Regulatory T Cells (Treg) are the subgroups of lymphocytes that control inflammatory responses and regulate homeostasis processes through cellular contact and the secretion of soluble agents. We investigated the frequency of Treg in peripheral blood of sulfur mustard exposed patients with CD4, CD25, CD127, FOXP3, and CD45RA markers, compared with healthy individuals and the correlation between frequency of these cells and long-term pulmonary complications. Materials and Methods: In total, 12 sulfur mustard exposed patients and 12 healthy volunteers were invited. Clinical inspections of both groups were performed by pulmonary specialists and spyrometric evaluation that pulmonary function test operators conducted. After the isolation of peripheral blood mononuclear cells, the frequency of Treg cells was determined by flowcytometry. Results: The frequency of Treg cells was not significantly different in those exposed to sulfur mustard. Furthermore, there was no significant correlation between spyrometric parameters and the frequency of these cells. Conclusion: According to the current study data, there is no difference in the frequency of Treg cells between exposed patients with mild pulmonary complications and healthy volunteers. Thus, further studies are required to understand the role of these cells in the severity of pulmonary complications of these patients.