Between September 1, 1989, and August 31, 1990, 516 patients were admitted to the Royal Brompton National Heart and Lung Hospital for thoracic operations. A prospective audit recorded the nature and extent of operation, the histologic diagnosis, and the number of units of blood prepared and transfused during hospitalization. Cross-matched blood was requested in 243 patients but only 16.1% of these received transfusion. In total, 1,295 units of whole blood or red cell concentrate were cross-matched and made immediately available in the operating suite at the time of operation. Only 322 units were administered (cross-match to transfusion ratio of 4.02:1). Almost half of the patients who received transfusions received 2 units or less, a third received 3 or 4 units, 10% between 5 and 10 units, and 8.4% required more than 10 units during their hospital stay. The nature and extent of resection was an indicator of the need for transfusion. Other important predisposing factors included a previous thoracic operation, resection for inflammatory disease, decortication of empyema thoracis, chest wall resection, or thoracoplasty. Other thoracic procedures such as pleurodesis, pleurectomy, open lung biopsy, pectus correction, operation for bullous lung disease, and mediastinoscopy had a negligible transfusion requirement. The data suggest that understanding risk factors for transfusion requirements of patients undergoing thoracic surgical procedures should optimize present resources. This is critical when exploiting the limited availability of donated blood and blood products. Similarly, anticipation of transfusion requirements takes best advantage of manpower within the blood bank and minimizes unnecessary and avoidable blood wastage and expenditure.