Sulfur mustard (SM) is highly toxic for various organs. The eyes, skin, respiratory tract, as well hematopoietic and immune systems are the main organs affected by SM. Granulocyte-macrophage colony stimulating factor (GM-CSF) is a potent cytokine that plays an important role in the hematopoietic and immune system. The aim of this study was to determine the serum levels of GM-CSF and its relation to blood cell count and other inflammatory cytokines 20 years after SM exposure. The association of GM-CSF with the clinical severity of pulmonary, ophthalmic and dermatologic complications has also been studied. In this historical cohort study named as Sardasht-Iran Cohort Study (SICS), 369 SM exposed male participants and 125 unexposed volunteers were studied. The serum concentrations of cytokines were measured by ELISA technique. The severity of clinical complications was graded according to the criteria verified by the Medical Committee of the Foundation of Martyr and Veterans Affairs. The serum levels of GM-CSF in the SM exposed group did not display any significant differences with the control group. Median of GM-CSF was 7.33 and 9.39 pg/ml in the SM exposed group and the controls respectively. There was a positive correlation between the serum levels of GM-CSF and the percent of eosinophils only in the exposed group. Moreover, positive correlations were found between circulating levels of GM-CSF with IL-1alpha, IL-1beta, TNF-alpha and IL-6. This correlation was not observed between GM-CSF and IL-8 in both study groups. The serum levels of GM-CSF did not show any significant association with clinical complications.