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Previous studies have reported erythrocyte macrocytosis in adults and children with Down syndrome (DS), the significance of which remains unclear. We compared hematological parameters of 50 DS children aged 2 to 15 years, divided into three age groups, with those of 68 aged-matched healthy children. Patients with DS had a significantly increased mean corpuscular volume (MCV) and hemoglobin in all groups when compared with the controls. Erythrocyte creatine content, hexokinase (Hk) activity, erythrocyte and serum folates, vitamin B12, haptoglobin, serum iron, and ferritin were tested. All of these parameters were not significantly different from those of the control group. We conclude that macrocytosis may not be an expression of reduced red cell survival but rather of an altered folate remethylation pathway, secondary to enhanced cystathionine β-synthase (CBS) activity, the gene for which is present on chromosome 21.