We investigated whether pediatric patients with sickle cell disease (SCD) (9 +/- 4 years; 27 homozygous SCD [HbSS]; 19 sickle-C disease [HbSC]) have different folate status compared with age-, sex-, and race-matched normal hemoglobin (HbAA) controls (n = 20), and whether their folate status can be improved by folate supplementation. The patients were supplemented with vitamins B6 and B12 during one week and with folate during the following week. Circulating folate, homocysteine, vitamin B6 and vitamin B12 levels were measured at baseline (patients and controls), after one week and after two weeks (patients). The patients had similar folate, vitamin B6, and vitamin B12, but higher homocysteine levels compared with HbAA controls (12.7 +/- 4.5 vs. 10.9 +/- 3.5 micromol/l; P = 0.04). Vitamin B6 and B12 supplementation did not change their homocysteine levels, but folate supplementation caused a 53% reduction (to 5.7 +/- 1.6). We conclude that patients with SCD have adequate vitamin B6 and B12 status, but suboptimal folate status, leading to elevated plasma homocysteine levels. They may therefore benefit from folate supplementation to reduce their high risk for endothelial damage.