BACKGROUND The aim of this investigation was twofold: first, to demonstrate an association between endometriosis, ABO blood groups and Rhesus factor and, second, to show potential correlation of ABO blood group and stages of endometriosis. METHODS Two hundred and thirty-one women with endometriosis and 166 infertile women without endometriosis were studied retrospectively at the Yale University Hospital. All the cases were diagnosed by laparoscopy and in all of them ABO blood groups and Rhesus factor were determined by standard techniques. Women with endometriosis were divided into two groups according to the stage: Group 1 included 124 cases with stages I and II, and Group 2, 107 women with stages III and IV. Statistical methods included chi(2) and odds ratios (95% CI). RESULTS The identified distribution of ABO and Rh blood groups in women with endometriosis differed significantly from that of the women without endometriosis [chi(2) = 26.27, (P < 0.001); chi(2) = 18.71, (P < 0.001), respectively]. The blood group A was more predominant in women with endometriosis, while blood group O was less predominant. The overall risk of women with endometriosis and A blood group was 2.89 (95%CI, 1.85-4.52). No significant difference was detected in ABO and Rh blood groups in women with endometriosis according to the severity of disease. CONCLUSIONS Women with endometriosis have a 2.9-fold increased risk in the A blood group distribution. The role of blood groups in the development of endometriosis remains to be determined.