Congenital hemifacial hypertrophy is expressed in facial asymmetry resulting from unilateral overgrowth. We examined proliferative activity of primary cultured osteoblasts taken from the hemifacial hypertrophic side of an 18-year-old woman compared with the normal side. Both fetal calf serum and basic fibroblast growth factor stimulated osteoblast DNA synthesis, and cultured osteoblast proliferation was markedly increased in the hypertrophic side. Neutralizing antibody against basic fibroblast growth factor was effective to partially block fetal calf serum-induced DNA synthesis. These results suggest that fibroblast growth factor and its receptor signal transduction axis may be selectively involved in affected osteoblasts, leading to progression of congenital hemifacial hypertrophy.