In this article we aimed to assess how Alzheimer's disease (AD), which is neurodegenerative, affects the linguistic performance of early, high-proficient bilinguals in their two languages. To this end, we compared the Picture Naming and Word Translation performances of two groups of AD patients varying in disease progression (Mild and Moderate) with that of bilingual individuals diagnosed with mild cognitive impairment (MCI). The results revealed that the linguistic deterioration caused by AD affected the two languages similarly. We also found that cognate status and word frequency were two major determinants of language performance in all three groups of participants. These results are consistent with the notion of a common neural substrate recruited to represent and process the two languages of high-proficient bilinguals. (Contains 4 figures and 5 tables.)