Background and Purpose. Only a few attempts have been made to establish the impact of critical intima-media thickness (IMT) on narrowing of the lumen of the common carotid artery (CCA). In the present study, sonomorphological criteria have been used to assess how intima-media thickening in the CCA may influence the artery geometry. Methods. High-resolution ultrasonography was employed in 233 patients (466 arteries) to quantify the selected parameters of CCA biometry: IMT, arterial lumen diameter (LD), interadventitial diameter (IAD), and outer artery diameter (OAD). Results. With an increase of CCA IMT up to the critical point of 1.2 mm, the LD showed parallel compensatory increases. Above the inflection point of 1.3 mm, the lumen became progressively narrower proportionally to the increasing IMT. Conclusion. There are limits to the compensatory enlargement of the CCA lumen. Above the inflection point of CCA IMT of 1.3 mm, the artery lumen becomes progressively narrower with increasing IMT.