Introduction: Oral ulcers are one of the most common oral lesions and chief complaints of the patients referred to the Oral Medicine department. This study was conducted to determine the prevalence of different etiologies of oral ulcers in oral medicine ward of Mashhad university of medical sciences Materials and Method: This retrospective and descriptive study was conducted on 294 patients referred to the oral medicine ward of Mashhad dental school between 2002 and 2007 due to oral ulcers. Required data were collected from medical records of patients and analyzed by descriptive statistics. Results: Aphthous ulcers were the most common oral ulcers (27.9%). Oral ulcers were more prevalent in females. Tongue was the most common site involved (44.21) and oral ulcers were most prevalent between the ages 21-40. Multiple ulcers were more common (76%). Conclusion: Oral ulcers are among the most common complaints of patients referred to the Oral Medicine department. Due to geographic variations, epidemiologic studies can be helpful in diagnosis of these lesions.