Anaemia is common before cardiac surgery and is associated with increased morbidity and mortality. The World Health Organization (WHO) definition of anaemia is lower for women than for men by 10 g.l-1 , potentially putting women at a disadvantage compared with men with regard to pre-operative optimisation. Our hypothesis was that women with borderline anaemia (defined by us as haemoglobin concentration 120-129 g.l-1 ) would have a higher rate of red cell transfusion, morbidity and mortality than non-anaemic women (haemoglobin ≥ 130 g.l-1 ). This retrospective observational study included all adult patients admitted for elective cardiac surgery from January 2013 to April 2016. During the study period, 1388 women underwent cardiac surgery. Pre-operatively, 333 (24%) had a haemoglobin level < 120 g.l-1 ; 408 (29%) 120-129 g.l-1 ; and 647 (47%) ≥ 130 g.l-1 . Compared with non-anaemic women, women with borderline anaemia were more likely to be transfused (68.6% vs. 44.5%; RR 1.5, 95%CI 1.4-1.7; p < 0.0001) and were transfused with more units of red cells, mean (SD) 1.8 (2.8) vs. 1.3 (3.0); p < 0.0001. Hospital length of stay was significantly longer in the borderline anaemia group compared with non-anaemic women, median (IQR [range]) 8 (6-12 [3-45]) vs. 7 (6-11 [4-60]); p = 0.0159. Short- and long-term postoperative survival was comparable in both groups. Borderline anaemia is associated with increased red cell transfusion and prolonged hospital stay. Future research should address whether correction of borderline anaemia results in improved outcomes.