Objective: To assess the possibility of a direct effect by areca nut extract on cell proliferation and endothelin production through the analysis of cultured keratinocytes from normal buccal mucosa and oral submucous fibrosis (OSF) buccal mucosa in vitro. Methods: Keratinocytes were isolated and cultured with or without areca nut extract in the presence of 10% fetal calf serum for 48 hours at 37°C in 5% CO₂ and air. Using radio immunoassay (RIA), endothelin was quantified in culture supernatants. The cell proliferation was monitored by thiazolyl blue assay. Results: Areca nut extract inhibited cell proliferation in a concentration-dependent manner. Concentration of endothelin in oral submucous fibrosis keratinocyte culture supernatants was higher than that found in normal keratinocytes. Endothelin quantities were significantly elevated (P < .05) when keratinocytes were exposed to areca nut extract. Conclusion: The findings suggest that areca nut extract can inhibit cell proliferation, and the increase of endothelin production may play a role in the pathogenesis of oral submucous fibrosis.