In eight patients with sickle cell anemia, weekly extracorporeal carbamylation of about 20% of the circulating red cell mass was carried out for 2 yr or longer. At each visit, a mean of 1.3+/-0.2 mol of cyanate were incorporated per mole of hemoglobin in the carbamylated erythrocytes. Within 3 mo, a stable level of about 35-50% of the circulating erythrocytes was carbamylated. This quantity and degree of hemoglobin carbamylation produced a decrease in mean whole blood P50 from 33 to 26 mm Hg. During the first 3 mo of carbamylation, the mean hemoglobin increased from 6.4 to 9.1 g/100 ml, while mean absolute reticulocytes decreased by 58% and circulating irreversibly sickled erythrocytes decreased by 65%. The mean red cell life span increased from 13 days before treatment to 21.6 days after 3 mo of carbamylation. Beyond the 3rd mo of carbamylation, blood P50, hemoglobin, and reticulocytes remained quite stable. No toxic effects of extracorporeal carbamylation of erythrocytes were noted. The capacity of blood to release oxygen at 30 mm Hg PO2 increased from 4.3 to 5.0 cm3/100 ml blood during carbamylation. The overall frequency of severe painful crises decreased by about 80% during carbamylation. Before carbamylation, 34% of the crises were induced by a concomitant illness, usually an infection. During carbamylation, the incidence of induced crises decreased 50% while spontaneous crises virtually disappeared. The marked improvements in hematologic parameters and the decreased frequency of severe painful crises observed during this study offer sufficient promise to warrant further exploration, hopefully using more efficient techniques, of the clinical efficacy of extracorporeal erythrocyte carbamylation in sickle cell anemia.