Abstract Intima-media thickness (IMT) has been measured for over 20 years, and is widely regarded as a surrogate for atherosclerosis. However, in the carotid arteries atherosclerosis is focal, manifesting as plaques. IMT is often measured deliberately where no plaque exists, or multiple measurements may be averaged, including only one or two that intersect plaque. IMT and plaque are biologically and genetically distinct, so they can be expected to respond differentially to therapies for atherosclerosis. Furthermore, because plaques grow along the carotid arteries 2·4 times faster than they thicken, progression or regression of total plaque area is more sensitive to effects of therapy than IMT. Because plaques also grow and regress circumferentially, three-dimensional (3-D) plaque volume is two orders of magnitude more sensitive to effects of therapy than is IMT. While 3-D ultrasound requires special equipment, total plaque area can be measured using the same equipment as IMT. Because plaque and IMT are biologically and genetically distinct entities, representing different phenotypes of atherosclerosis, both should be measured in any situation where IMT is measured, with the exception of studies in children too young for the occurrence of plaque. IMT should not be called ‘atherosclerosis’: the phenotype being assessed should be specified.