The aim of this retrospective study was to evaluate the relationship between HELLP (hemolysis, elevated liver enzymes, low platelets) syndrome and the maternal blood groups. Five hundred and forty-seven women with severe preeclampsia were included and divided into eight groups according to their blood groups: A Rh-positive (n=203), A Rh-negative (n=38), B Rh-positive (n=83), B Rh-negative (n=10), 0 Rh-positive (n=148), 0 Rh-negative (n=21), AB Rh-positive (n=39), and AB Rh-negative (n=5). The groups were controlled by analysis of variance and found to be homogeneous with respect to parity, gestational age, blood pressure, hemoglobin, hematocrit, platelet values, prothrombin time, partial thromboplastin time, fibrinogen, creatinine, alanine aminotransferase, aspartate aminotransferase, bilirubin, uric acid, and proteinuria. Incidence of HELLP syndrome was 24% in the overall study population whereas 48% of the patients with the blood group O Rh-negative had HELLP syndrome associated with an increase in risk by a factor of 3.1. To our knowledge this is the first report of such an association.