Objetives: Recurrent aphthous stomatitis (RAS) is a common pathology of the oral mucosa with a complex and multifactorial etiology. Tumour Necrosis Factor-alpha (TNF?) is a cytokine with an important but not well-known role in the development of new lesions in RAS patients. Modifications of salivary levels of TNF? in RAS patients during the active periods of the disease have been measured in this work. The possible implication of TNF? in RAS etiology is also discussed. Study design: The study group was composed of 20 patients previously diagnosed with RAS and randomly selected. As a control group 10 healthy patients were also randomly selected. In both groups a TNF? assessment was carried out in non stimulated saliva. All the patients in the study group presented active lesions at the moment of the salivary sample collection. Values oscillating between 0 and 8.1 pg/ml were considered as normal. Results: Salivary TNF? levels are 2 to 5 times higher in RAS patients than those of healthy patients. Conclusions: TNF? has a possible implication in the RAS etiology and it may also have an important role in the search of new treatments for this disease.