Subclinical hypothyroidism (SH), defined as increased TSH serum levels and normal serum free T4 concentrations, has been associated with an increased risk of coronary heart disease in adults. Data in children and adolescents are scanty. The objective of the study was to investigate the clinical and biochemical cardiovascular risk factors in children with mild SH (serum TSH concentrations 4.5-10 mU/L). This is a cross-sectional and controlled study conducted at a tertiary referral center on patients with persistent idiopathic long-standing (3.2 ± 0.4 y) mild SH. At study entry patients and controls underwent a clinical and biochemical assessment for cardiovascular risk. Forty-nine children aged 8.5 ± 0.5 years with SH and 49 controls were enrolled in the study. Systolic and diastolic blood pressure, body mass index (BMI), waist to height ratio, lipid profile, homocysteine, high-sensitivity serum C-reactive protein, fibrinogen, adiponectin, insulin, and homeostasis model assessment index were measured. Waist to height ratio (P < .0001), atherogenic index (P = .001), triglycerides to high-density lipoprotein-cholesterol ratio (P = .01), and homocysteine levels (P = .002) were significantly higher and high-density lipoprotein-cholesterol significantly lower (P = .003) in SH subjects compared with controls. No significant differences were found in the other clinical and biochemical cardiovascular risk factors analyzed. Multivariate regression model revealed that BMI and thyroid status were the main independent factors affecting dependent variables. Even after an adjustment for BMI, most of the variables still remained significantly associated with mean TSH levels or SH duration. Mild long-lasting untreated idiopathic SH may be associated with subtle proatherogenic abnormalities. Although it is difficult to establish whether these mild abnormalities represent the early steps in the initiation of atherogenesis, these children need to be carefully monitored for metabolic complications.