From observations on 454 coronary arteries from subjects ranging in age from prematurely newborn to 90 years, six structural patterns of medial smooth muscle are interpreted as representing, or related to, the proliferation and/or migration of medial smooth muscle into intima. By measuring the extent of inner medial circumference occupied by four of the six patterns, it was possible to calculate a numerical value designated the medial proliferatice and / or migratory activity (MP-MA) of each artery. During the first three decades, nonatherosclerotic diffuse intimal thickening was the characteristic intimal process, and during this phase of the arterial maturation span, the MP-MA of the arteries was significantly related to the degree of intimal thickening. Following a peak MP-MA level by the end of the third decade, there was a progressive decrease in the MP-MA level as the incidence and severity of atherosclerotic intimal thickening increased. At advanced stages of atherosclerotic intimal thickening, which were associated with thinning of adjacent media, intimal-medial structural patterns indicating a relationship between medial smooth muscle proliferative activity and the expanding atherosclerotic plaque were also apparent. The observations support the concept that the movement of medial smooth muscle into intima is a critical step preceding and during the evolution of the atherosclerotic plaque.