The present paper deals with first language attrition under the influence of a second language. Two sets of data are presented: the first is offline data from a variety of tasks aiming to address the question of selective vulnerability of interfaces as opposed to the computational system underlying syntactic competence. This data is drawn from the study of Tsimpli, Sorace, Heycock and Filiaci (2004). The second set of data is drawn from a pilot study (Kaltsa 2006), involving an online grammaticality judgment task testing Case on determiners. This data is argued to address the possibility of syntactic attrition of a formal feature, i.e. case, in the domain of syntactic processing alone indicating that performance rather than competence problems are also involved.