Objectives: To determine the prevalence of oral leukoplakia and erythroplakia in a group of Thai patients and to study the clinical and pathologic features of these 2 lesions. Method and Materials: Cases having provisional diagnoses of leukoplakia and erythroplakia between 1973 and 2004 were retrieved from the files of the Department of Oral Pathology, Faculty of Dentistry, Mahidol University. Clinical and pathologic features were reviewed and analyzed. Results: Of 7,177 biopsy specimens, 123 cases (1.7%) of leukoplakia and 9 cases (0.13%) of erythroplakia were found. There was no significant gender predilection in leukoplakia (male:female = 1.2:1). The peak of age-frequency distribution of leukoplakia was in the fourth decade among men and fifth decade among women. The most common site of leukoplakia was buccal mucosa (28.5%), followed by alveolar mucosa (18.7%) and tongue (16.3%). Microscopic study of leukoplakia revealed hyperkeratosis with or without acanthosis in 60.9% of cases, epithelial dysplasia in 10.6%, and squamous cell carcinoma in 4.9%. Erythroplakia was found in 6 men and 3 women. It was most frequently seen during the seventh decade of life. The palate was the most common site. Epithelial dysplasia and squamous cell carcinoma were found in 6 patients with erythroplakia (66.7%). Conclusions: Leukoplakia occurred 13 times more frequently than erythroplakia. However, squamous cell carcinoma was more frequently found in erythroplakia cases. Both lesions were found most frequently in elderly individuals and affected men more than women. Keywords: clinicopathologic study, erythroplakia, leukoplakia, Thai