To demonstrate the role of first language proficiency in second-language acquisition and lower school success, this paper proposes the findings of an empirical study collating bilingual Turkish immigrant children (n = 30, mean age = 67.27 months) in the Netherlands and monolingual Turkish speakers (n = 30, mean age = 66.97 months) in Turkey. Collated to monolingual Turkish speakers, Turkish immigrant children lag behind in their first language cognitive notions, lexical, syntactic, and textual skills. Mothers’ education level appears a notable factor in accounting immigrant children's performance contrasts. This would affect their concept and cognitive development and their second-language acquisition.