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.