In order to find the effect of genetic polymorphisms of glutathione S-transferase M1 (GSTM1) and GSTT1 on hematological changes of individuals chronically exposed to natural sour gas, the present study was done. Study subjects (59 males, 55 females) were residents of contaminated areas of Masjid-i-Sulaiman (southwest of Iran). The GSTM1 and GSTT1 genotypes were determined using a polymerase chain reaction-based method. The multiple linear regression method was applied. There is significant association between GSTs genotypes and either hemoglobin (t = 2.185, P = 0.031) or hematocrit (t = 2.454, P = 0.016). Also there is weak association between GSTs genotypes and WBC counts (t = 1.802, P = 0.074). The hemoglobin and hematocrit levels and WBC counts increased in individuals who had null genotypes of GSTM1 and GSTT1 compared to subjects with one or two active genes. Also the levels of hemoglobin and hematocrit and WBC counts increased in persons with one active genotype compared to subjects who had two active genes. There is no significant association between neither platelet nor WBC differential parameters and GSTs genotypes. (C) 2004 Elsevier Inc. All rights reserved.