About 7% of the adult population has subclinical cobalamin (B12) deficiency. Subjects with sickle cell disease (SCD) may be at higher risk of cobalamin deficiency because of increased demand, inadequate supply, coexisting folate deficiency or malabsorption. We compared the clinical and laboratory characteristics of low serum cobalamin levels in patients with SCD with those patients without this hemoglobinopathy (non-SCD). Between 1993 and 2003, 105 SCD patients and 112 non-SCD patients who had serum cobalamin measurements were identified at our institution. The mean cobalamin level in SCD patients was significantly lower (496 +/- 352 pg/ml) than that in patients without SCD (869 +/- 660 pg/ml, p<0.0001). The frequency of low cobalamin levels, defined by a serum cobalamin level of <200 pg/ml, was 18.1% (19/105) and 9.8% (11/112) in SCD and non-SCD patients, respectively (chi2=3.11, nonsignificant). The mean age of the low-cobalamin SCD and non-SCD patients was 28.1 and 62.9, respectively, and their male:female ratios were 11:8 in SCD patients and 2:9 in non-SCD patients. None of the SCD patients had neurological manifestations, but nine of the 11 non-SCD low-cobalamin level patients did. The proportion of SCD patients with unexplained low cobalamin levels (13/19) was higher than that in non-SCD patients (4/11, chi2=2.92, nonsignificant) Our data suggest that cobalamin levels are lower in SCD patients than in subjects without SCD, and low-cobalamin SCD patients are younger and more likely to be males.