Six patients with sickle cell anemia were treated with sodium cyanate (30 mg/ kg/day). In four, treatment was stopped because of definite or suspected toxicity, and no improvement was seen in the other two. Most alarming was the sudden development of a peripheral motor neuropathy in a patient whose red blood cells contained less than 0.6 mols NCO /mol of hemoglobin; six months after treatment was stopped, function had not completely returned in this patient. Safe oral dosage regimens may not be effective, but extracorporeal treatment of sickle cells with cyanate, or other compounds, might circumvent that problem.