OBJECTIVE To evaluate the possible association of the glutathione S-transferase M1 (GSTM1) gene polymorphism with the susceptibility to endometriosis in women of Han nationality in Guangdong Province. METHODS Polymerase chain reaction was used to identify the GSTM1 genotypes in 76 patients with endometriosis and 80 controls (surgical patients for gynecological problems other than endometriosis). RESULTS The frequencies of the GSTM1 null genotypes in patients with endometriosis and controls were 65.8% and 46.3%, respectively, showing a significant difference between the endometriotic cohort and the control group (X(2)=6.03, P < 0.05). Individuals with GSTM1 null genotype were exposed to risks for endometriosis 2.24 times that of subjects without these genotypes OR=2.24, 95% CI=1.17-4.27 . CONCLUSIONS GSTM1 gene deletion might bea risk factor for endometriosis in women of Han nationality who are native residents in Guangdong Province.