Avascular necrosis (AVN) of the humeral and femoral heads is a frequent and debilitating complication of sickle cell disease. Some of the risk factors for AVN are alpha-thalassemia and age. Recently, newly discovered thrombophilia mutations have been associated with AVN in patients without sickle cell disease. We studied the frequency of the thermolabile methylene tetrahydrofolate reductase (MTHFR) variant (C677T) in adult sickle cell patients with and without AVN. The frequency of the MTHFR mutation was 35.6% in patients with AVN and 12.9% in those without AVN (p = 0.006). These data suggest that the thermolabile MTHFR variant may be a contributing risk factor for AVN in some populations with sickle cell disease.