Costello syndrome is a connective tissue disorder associated with sparse, thin, and fragmented elastic fibers in tissues. In this study we demonstrated a significant decrease in the expression of tropoelastin mRNA in fibroblasts derived from a Japanese Costello syndrome patient with impaired elastogenesis and enhanced proliferation. In contrast, there were no changes in expression of the Harvey ras (HRAS), fibrillin-1, fibulin-5, microfibril-associated glycoprotein-1 (MAGP-1), lysyl oxidase (LOX), or 67-kDa non-integrin elastin-binding protein (EBP) gene. The proliferative activity of the Costello fibroblasts was about 4-fold higher than that of the normal and pathological control ones. However, no mutations were detected in the coding region of HRAS mRNA. Transduction of the bovine tropoelastin (bTE) gene with the lentiviral vector restored the elastic fiber formation and decreased the growth rate in the Costello fibroblasts. These results strongly suggest that the defect of human tropoelastin (hTE) gene expression should induce the impaired elastogenesis and enhanced proliferation of Costello fibroblasts, and that a primary cause other than the HRAS gene mutation should contribute to the pathogenesis in the present Costello case.