OBJECTIVES: Clinical and epidemiological findings have provided evidence supporting a role of free radicals in the etiology of cancer. Scavengers and inhibitors of free radical processes have been demonstrated to prevent or delay the neoplastic process. PATIENTS AND METHODS: Adenosine deaminase, xanthine oxidase, superoxide dismutase, and glutathione peroxidase activities and malondialdehyde levels were measured in the sera of 35 patients with head and neck cancers and were compared to those of healthy control subjects. RESULTS: Serum adenosine deaminase activity was found to be significantly increased in the patient group (p<0.001). Compared to the control group, glutathione peroxidase and xanthine oxidase activities and malondialdehyde levels were slightly higher and serum superoxide dismutase activity was slightly lower in the patient group, with none reaching statistical significance. CONCLUSION: The results indicate that serum adenosine deaminase activity may be helpful in the diagnosis and follow-up of head and neck cancers. Further studies with a larger cohort of patients are needed to clarify the exact mechanism of adenosine deaminase elevation.