The interdependence between the first and second language of bilingual immigrant children has not received sufficient attention in research. Most studies concentrate on mainstream language skills of immigrant pupils. In some studies, the gaps in the language development of immigrant children are documented by comparing mainstream pupils with immigrant children. The competence in the first language receives very little attention. In order to show the role of first language competence in second-language acquisition and lower school achievement, we present the findings of our empirical study by comparing 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. The evidence presented in this paper shows that compared to monolingual Turkish speakers, Turkish immigrant children lag behind in their first language cognitive concepts, lexical, syntactic, and textual skills. Mothers’ education level turns out to be an important factor in explaining the performance differences of immigrant children. This would have an effect on their concept and cognitive development as well as on their second-language acquisition.