To investigate relationships between carotid arterial intima-media thickness (IMT) and age in childhood, we performed high-resolution carotid arterial ultrasonography in 60 healthy children (27 boys, 33 girls; age range, 5-14 years) determined by screening to have no dyslipidemia or hypertension. No plaque formation was found, and irregularity of IMT (root mean square roughness of IMT) did not correlate with age. Mean IMT increased in a linear manner with age [IMT in millimeters = (0.009 x age in years) + 0.35] ( r = 0.39, P = 0.002). This correlation remained significant after adjustment for gender, parental smoking, systolic and diastolic blood pressure, body mass index, and serum concentrations of low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides. None of these known cardiovascular disease risk factors in adults had a significant relationship with age-adjusted IMT in children. While circumferential wall stress and diastolic blood pressure were not correlated with age, mean IMT and lumen diameter showed significant positive relationships with circulating blood volume, which was calculated as the function of height and weight. These data suggested that age-dependent physiologic thickening of arterial walls begins in childhood.