Simvastatin has been shown to restore endothelial function in children with familial hypercolesterolemia after 28 weeks of treatment. The aim of this study was to evaluate 1-month simvastatin treatment effect on endothelial function in hypercholesterolemic children and adolescents. Eighteen hypercholesterolemic patients (HC group) and 18 healthy controls, aged 6-18 years, were studied with medical history, physical examination, full lipid profile, serum apolipoprotein B (apo B), fibrinogen, hepatic transaminases, and creatine kinase concentrations. Flow-mediated dilatation (FMD) was performed by high-resolution ultrasound of the brachial artery. The HC group received simvastatin 10 mg/day for 1 month. Arterial diameter was measured by two experienced sonographers who were unaware of subjects' conditions. At baseline, FMD was impaired in the HC group (mean, 5.27 ± 4.67%) compared to controls (mean, 15.05 ± 5.97%) (p < 0.001). After treatment, we observed a significant reduction in total cholesterol (TC) (29%), low-density lipoprotein cholesterol (LDL-C); (37%), apo B concentrations (36%) and FMD restoration (mean, 12.94 ± 7.66%), with an absolute increase of 7.66 ± 8.58 (p = 0.001). These results show that children and adolescents with hypercholesterolemia present endothelial dysfunction, and simvastatin, in addition to significantly reducing TC, LDL-C, and apo B concentrations, restores endothelial function with 1-month treatment.