Objectives Recurrent aphthous ulceration (RAU) is one of the most common oral mucosal disorders found in humans. Although the exact etiology of RAU is unkown, local and systemic conditions, and genetic, immunologic, and infectious factors all have been identified as potential etiopathogenic agents. The aim of our study was to compare serum xanthine oxidase (XO) and adenosine deaminase (AD) activities, and malondialdehyde (MDA), nitric oxide (NO) and uric acid (UA) levels in a group of patients affected by RAU and in a group of healthy controls. Subjects and Methods A total of 26 patients with minor RAU were included in the study. Twenty-six healthy volunteers were selected to form the control group. AD and XO activities, and UA, NO, and MDA levels were studied in the serum samples of all patients and controls. Results Serum XO and AD activities, and MDA, NO, and UA levels were significantly higher in RAU patients than in controls. Conclusions Increased XO and AD activities, NO and UA levels and lipid peroxidation were thought to take part in the pathogenesis of RAU. Hence the effects of XO inhibitors in the treatment of RAU should be evaluated in future studies.