Phenotypic modulation of smooth muscle cells (SMCs) plays an integral role in atherosclerosis, hypertension and leiomyogenic tumorigenicity. The morphological, functional, and biochemical characteristics of SMCs in different phenotypes such as differentiated and dedifferentiated states have been well studied. Recent researches have focused on the expressional regulation of SMC-specific marker genes in association with phenotypic modulation of SMCs. The SMC-specific marker genes are regulated at the levels of transcription and splicing. The caldesmon, smooth muscle myosin heavy chain, α-smooth muscle actin, calponin, SM22, α- and ß-tropomyosins, and αl integrin genes are transcriptionally regulated; transcription of these genes except for the α-smooth muscle actin gene is upregulated in differentiated SMCs, but is downregulated in dedifferentiated SMCs. The expression pattern of a-smooth muscle actin is opposite in vascular and visceral SMCs. In almost all promoter regions of these genes, the CArG box and serum response factor (SRF) are involved in as the positive cis-element and the trans-acting factor, respectively. Isoform changes of caldesmon, α-tropomyosin, vinculin/metavinculin, and smooth muscle myosin heavy chain are regulated by alternative splicing in a SMC phenotype-dependent manner. Among them, isoform interconversions of caldesmon and α-tropomyosin are completely coordinated with phenotype of SMCs. The purpose of this paper is to summarize current knowledge of the expressional regulation of SMC-specific marker genes in different phenotypes of SMCs. (Mol Cell Biochem 190: 105–118,1999)