Background: In the literature so far the limited research on specific language impairment (SLI) in bilingual children has concentrated on linguistic skills in the first language (L1) and/or the second language (L2) without paying attention to the relations between the two types of skills and to the issue of linguistic transfer. Aims: To examine the first and second language proficiency of 75 Turkish-Dutch bilingual children with SLI in the age range between 7 and 11 years living in the Netherlands. A multidimensional perspective on language proficiency was taken in order to assess children's Turkish and Dutch proficiency levels, whereas equivalent tests were used in order to determine language dominance. A second aim was to find out to what extent the children's proficiency in L2 can be predicted from their L1 proficiency, while taking into account their general cognitive abilities. Methods & Procedures: The children's performance on a battery of equivalent language ability tests in Turkish and Dutch was compared at three age levels. By means of analyses of variance, it was explored to what extent the factors of language and grade level as well as their interactions were significant. Bivariate correlations and partial correlations with age level partialled out were computed to examine the relationships between L1 and L2 proficiency levels. Moreover, regression analysis was conducted to find out to what extent the variance in general L2 proficiency levels could be explained by children's L1 proficiency, short-term memory and non-verbal intelligence. Outcomes & Results: Repeated measures analyses showed that the children had generally higher scores on L1 as compared with L2 and that with progression of age the children's scores in L1 and L2 improved. Medium to high correlations were found between phonological memory, phonological awareness, grammatical skills and story comprehension in the two languages. Regression analysis revealed that children's L2 proficiency levels could be explained by their proficiency levels in L1, even after controlling for children's non-verbal intelligence and working memory. Conclusions & Implications: It is concluded that children's formal linguistic skills in L1 and L2 tend to be related and that their level of L1 proficiency may help to develop linguistic skills in L2. [ABSTRACT FROM AUTHOR]