Plasma soluble VCAM level above median value for sickle cell patients defined by assays performed in our laboratory on the blood of sickle cell patients seen in our program or tricuspid regurgitant jet velocity (determined by Doppler echocardiography) greater than 2.4 m/sec or has had a subnormal response to L-NMMA or sodium nitroprusside infusion on a previous forearm blood flow study.