Oral leukoplakia (OL) is the most common premalignancy in the oral cavity. A small proportion of OLs progresses to oral squamous cell carcinoma (OSCC). To assess OSCC risk of OLs, we investigated the role of the transcriptional repressor enhancer of zeste homolog 2 (EZH2) in oral tumorigenesis and its clinical implication as an OSCC risk predictor. Immunohistochemistry was used to measure EZH2 expression in OLs from 76 patients, including 37 who later developed OSCC and 39 who did not. EZH2 expression was associated with clinicopathologic parameters and clinical outcomes. To determine the biological role of EZH2 in OL, EZH2 level was reduced using EZH2 siRNAs in Leuk-1 cells, its impact on cell cycle, anchorage-dependent/independent growth, and invasion was assessed. We observed strong EZH2 expression in 34 (45%), moderate expression in 26 (34%), and weak/no expression in 16 (21%) of the OLs. The higher EZH2 levels were strongly associated with dysplasia (P < 0.001) and OSCC development (P < 0.0001). Multivariate analysis indicated that EZH2 expression was the only independent factor for OSCC development (P < 0.0001). At 5 years after diagnosis, 80% of patients whose OLs expressed strong EZH2 developed OSCC whereas only 24% patients with moderate and none with weak/no EZH2 expression did so (P < 0.0001). In Leuk-1 cells, EZH2 downregulation resulted in G(1) arrest; decreased invasion capability, decreased anchorage-independent growth; downregulation of cyclin D1 and upregulation of p15(INK4B). Our data suggest that EZH2 plays an important role in OL malignant transformation and may be a biomarker in predicting OSCC development in patients with OLs.