Low apoA-I level is a marker for endothelial dysfunction. Forearm blood flow was measured in 26 patients with SCD with venous occlusion plethysmography following test doses of acetylcholine (ACh) and sodium nitroprusside (SNP) infused into the brachial artery. Patients with higher than median levels of apoA-I () demonstrated dose-dependent vasodilation to ACh close to previously published normal values. In sharp contrast, those with lower than median levels of apoA-I had markedly blunted responses measured as (A) absolute blood flow (P < .001, 2-way ANOVA) or as (B) percentage of increase from baseline (P = .001). Although the (C) absolute blood flow was lower at baseline and all doses of SNP in low apoA-I patients (P = .007), the (D) percentage increase from baseline did not differ by apoA-I status.