Open-heart surgery has been performed since 1975 on 25 patients who are Jehovah's Witnesses by religion. The patients' ages ranged from 6-60 years, and their body weights from 18-51 kg. Surgical procedures included correction of congenital heart disease in 14 patients and valve repair or replacement in 11. Six procedures were reoperations. The lowest mean haematocrits, during perfusion and the postoperative period, were 22.7% (range 15.0-31.0%) and 27% (range 16.0-36.0%), respectively. Twenty-four patients survived and are alive and well. One patient died of low output failure before discharge. The blood return system reduced blood loss. Five of the patients who underwent cardiac surgery received recombinant erythropoietin before and after surgery, leading to higher postoperative haematocrits. In one patient, a haematocrit which fell to 16.9% after surgery was raised to 27% by administration of erythropoietin, without blood transfusion. In two recent cases, high doses of aprotinin were used during surgery, resulting in better haemostasis after cardiopulmonary bypass.