Recent studies suggest that persistent intravascular haemolysis, a defining feature of sickle cell disease (SCD), is associated with severe long-term consequences, including pulmonary hypertension, which carries a 2-year mortality rate of up to 50%(Castro et al, 2003). Thus, the development of therapies that correct intravascular haemolysis and its complications are increasingly recognized as important for the long-term prognosis of the SCD patient. Haematopoietic stem cell transplantation (HSCT) is a potentially curative therapy for SCD, and recent improvements in supportive care and the development of reduced-intensity conditioning regimens have substantially lessened the severity of the immediate toxicities of the transplant procedure (Locatelli, 2006). When lower intensity conditioning regimens are utilized, host haematopoiesis is incompletely eliminated, and mixed haematopoietic chimerism frequently results. The effects of persistent recipient erythro-poiesis on intravascular haemolysis are unknown, but would be anticipated to perpetuate intravascular haemolysis. Several serum biomarkers have been recently identified to strongly correlate with endothelial damage, pulmonary hypertension and prospective early mortality in SCD patients. These include increased soluble vascular cellular adhesion molecule 1 (sVCAM-1) levels, increased nitric oxide (NO) consumption, increased plasma free haemoglobin (Hb), and inverted arginine/ornithine ratio (Solovey et al, 1997; Reiter et al, 2002; Morris et al, 2005). Using these parameters, we examined the potential of partial donor engraftment to correct intravascular haemolysis in nine SCD patients, and hence to potentially avert the development of long-term SCD-associated complications.