Smooth muscle myosin heavy chains (MHC) exist in multiple isoforms. Rabbit smooth muscle contain at least three types of MHC isoforms; SM1 (204 kDa), SM2 (200 kDa) and SMemb (200 kDa). SM1 and SM2 are specific to smooth muscle, but SMemb is a nonmuscle-type MHC abundantly expressed in the embryonic aorta and in activated mesenchymal cells. We previously reported that these three MHC isoforms are differentially expressed in rabbit during normal vascular development and in experimental arteriosclerosis and demonstrated that MHC isoforms are excellent markers for smooth muscle phenotype. In order to clarify the clinical significance of MHC isoforms, this article will focus on the expression of smooth muscle MHC isoforms in normally developing and atherosclerotic human arteries, especially in coronary arteries. We recently isolated and characterized three cDNA clones encoding human SM1, SM2, and SMemb. The expression of SM2 mRNA in the human fetal aorta was significantly lower as compared to SM1 mRNA but the ratio of SM2- to SM1-mRNA was increased after birth. SMemb mRNA in the aorta was decreased after birth. Immunohistologically, SM1 was constitutively positive from the fetal stage to adulthood in the apparently normal media of the aorta and coronary arteries, whereas SM2 was not detected in fetal arteries of early gestational stage. SM2 was recognized in well-differentiated smooth muscle after perinatal stage. In the human aorta or coronary arteries, unlike in rabbit, SMemb was detected even in the adult. Mild diffuse intimal thickening in the major coronary arteries of the young was found to be composed of smooth muscle cells, reacting equally to three antibodies for MHC isoforms. In thickened but non-atheromatous intima, the expression of well-differentiated smooth muscle-specific MHC (SM2) was reduced, especially in the deeper layer. With progression of atherosclerosis, intimal smooth muscle diminished the expression of not only SM2 but also SM1, whereas alpha-smooth muscle actin was well preserved. We conclude from these results that smooth muscle MHC isoforms are important molecular markers for studying human vascular smooth muscle cell differentiation as well as the cellular mechanisms of atherosclerosis.