Down syndrome (DS) patients have an increased risk of developing pulmonary hypertension (PH). Increased plasma levels of asymmetric dimethylarginine (ADMA) may contribute to vascular dysfunction in adults with idiopathic pulmonary hypertension. Our goal was to test the hypothesis that DS patients with PH have higher plasma levels of ADMA than DS patients without PH. DS patients with definitive PH (n = 6) and DS patients with no evidence of PH (n = 12) were studied. Plasma levels of arginine, ADMA, and nitrite/nitrate (NOx; stable metabolites of nitric oxide (NO)) were measured. Plasma arginine concentration was lower (p < 0.05) in PH patients (23 ± 11 μM) versus non-PH patients (46 ± 24 μM). Plasma ADMA concentration was higher (p < 0.005) in PH patients (18.0 ± 4.2 μM) versus non-PH patients (8.6 ± 5.9 μM). Plasma NOx was lower (p < 0.05) in PH patients (4.5 ± 1.7 μM) versus non-PH patients (8.5 ± 7.3 μM). These results are consistent with ADMA contributing to lower NO production in DS patients with PH and suggest that ADMA levels may be a potential biomarker for PH in DS patients.