There is extensive evidence showing that bilinguals activate lexical representations in a non-selective way both when words are presented in isolation and in sentence contexts. Recent research has shown the existence of cross-language activation at the syntactic level as well. However, the extent to which the lexical and syntactic levels of representation interact during second language (L2) sentence processing, and how these interactions are modulated by L2 proficiency remain unclear. In this paper, we explore how native speakers of European-Portuguese (L1) who are learning English as an L2 at different levels of proficiency (intermediate vs. advanced) resolve relative clause (RC) syntactic ambiguities in their L2. European Portuguese and English native speakers were used as controls. Participants were asked to perform a sentence completion task, with cognates and noncognates critically embedded in the complex noun phrase (NP) preceding the RC, and which contained its antecedent. Results revealed that L2 learners, like English controls, preferred to attach the RC to the last host of the complex NP, regardless of L2 proficiency. Importantly, the cognate status of the complex NP modulated the results, although, contrary to our expectation, the presence of cognates induced less L1 syntax interference compared to noncognates.