Introduction:Evidence from previous studies suggests that bilingualism is one of the factors which contributes to cognitive reserve, by showing that bilingual individuals manifest symptoms of Alzheimer's disease (AD) significantly later than monolinguals.Other evidences revealed that bilinguals show greater amounts of atrophy or hypometabolism compared to monolinguals, despite having the same diagnosis of AD and similar symptoms.However, these previous studies included samples in which bilinguals and monolinguals showed differences in education, immigration and/or socioeconomic status.The aim of this work was to investigate the neural bases of these putative prote ctive effects of bilingualism.Methods: A sample of 68 patients with mild cognitive impairment (MCI), 26 bilinguals and 42 monolinguals, was included in the study.Whole-brain 3-D magnetic resonance images were collected in order to compare the gray matter volume of both groups.Results: Bilinguals and monolinguals showed no significant differences in sociodemographic variables nor cognitive pe rformance in neuropsychological testing.However, bilinguals showed less gray matter volume (more brain atrophy) than monolinguals in the following areas: left precuneus, left Wernicke's area, bilateral insula/Heschl's gyrus and bilateral cerebellar vermis.Conclusion: Bilinguals with the same age, level of education and manifestation of symptoms as monolinguals, show higher levels of gray matter atrophy than monolinguals in relevant areas for AD.This suggests that bilingualism acts as a protective factor against dementia, since it requires more brain atrophy in order to get to the same pathological state.