The transfusion laboratories of 32 cardiothoracic surgical centres for adults were surveyed to determine the donor blood requirement for open heart surgery in the United Kingdom. Details of the transfusion practice and the use of blood conservation techniques were sought from a representative senior cardiac anaesthetist at each centre. Suitable data were received from 24 transfusion laboratories (75%) and 29 anaesthetists (90%). The mean (SD) blood use was 5.07 (1.53) units per operation. Seven centres routinely transfused fresh frozen plasma to all patients postoperatively. Experience with autologous deposit (three centres), "cell separators" (four centres), and the reinfusion of shed mediastinal blood (four centres) was limited. Prebypass phlebotomy for postbypass reinfusion (14 centres) and the infusion of residual oxygenator blood (27 centres) were the conservation techniques most commonly applied. In only nine centres was a postoperative normovolaemic anaemia to a haemoglobin concentration of less than 100 g/l accepted. Applying blood conservation techniques more widely would help to maintain blood supplies and reduce morbidity and mortality related to transfusion.