['Blood group O has been significantly associated with increased placental malaria infection in primiparae and reduced risk of infection in multiparae in the Gambia, an area with markedly seasonal malaria transmission. This study analyses the association between ABO blood group phenotypes in relation to placental malaria pathology and birth outcomes in southern Malawi, an area with perennial malaria transmission.', 'A cross-sectional study of 647 mother/child pairs delivering in Montfort Hospital, Chikwawa District between February-June 2004 and January-July 2005 was undertaken. Maternal peripheral and cord blood samples were obtained at delivery. Placental tissue was obtained and malaria histology classified as active, past or no malaria infection. Birth anthropometry was recorded. ABO blood group was measured by agglutination.', 'In primiparae, blood group O was significantly associated with increased risk of active placental infection (OR 2.18, 95% CI 1.15-4.6, p = 0.02) and an increased foetal-placental weight ratio compared to non-O phenotypes (5.68 versus 5.45, p = 0.03) In multiparae blood group O was significantly associated with less frequent active placental infection (OR 0.59, 95% CI 0.36-0.98, p = 0.04), and a higher newborn ponderal index compared to non-O phenotypes (2.65 versus 2.55, p = 0.007). In multivariate regression parity was independently associated with increased risk of placental malaria (active andpast infection) in primiparae with blood group O (p = 0.034) and reduced risk in multiparae with the same phenotype (p = 0.015).'] Do aBO blood group phenotypes influence parity specific immunity to Plasmodium falciparum malaria in Malawian women?