Endometriosis is a disease that affects 10 to 15% of the women of reproductive age. It is characterized by the presence of endometrial-like tissues outside of the uterus. Some definitions claim that the functional ectopic tissue is sensitive to the action of hormones. Severity of endometriosis is defined according to a system proposed by the American Society for Reproductive Medicine, which is based on laparoscopic findings. A large number of genetic polymorphisms has been reported for CYP1A1, the gene that is responsible for enzymes involved in stage I detoxification of xenobiotics; this gene is located at 15q22-24, and encodes an isoenzyme that catalyzes the oxidation of polycyclic aromatic hydrocarbons present in phenolic compounds and epoxides. The aim of this study was to analyze the frequency of the MspI polymorphism and its relation to endometriosis. We obtained peripheral blood samples from 52 women with endometriosis (confirmed by laparoscopy) as well as 42 women without endometriosis (control group). In the case group, the women were between 25 and 35 years of age; the age range was between 25 and 57 years old in the control group. Molecular analysis was performed by polymerase chain reaction. We found a significant association (P = 0.039) between the polymorphic allele m1 and endometriosis (32.70%). In conclusion, this study showed that the m1 polymorphism is associated with endometriosis, and that W1/m1 and m1/m1 polymorphisms are more frequently observed in patients with infertility and severe endometriosis.