Abstract Second language proficiency may be related to first language acquisition (Ganschow & Sparks, 1991), but relatively little is known about the relation between first and second language grammatical proficiency in primary school children who are in their first stages of foreign language learning. This study aims to determine whether differences in Dutch and English vocabulary and Dutch grammar skills predict differences in English grammatical proficiency in Dutch speaking children who are in grade 4 in primary school. The selected participants are monolingual Dutch pupils ( N = 152), aged 9;0–10;0. To measure the children’s vocabulary the PPVT was used in Dutch (Schlichting, 2005) and in English (Dunn & Dunn, 2007). In addition, two grammar tasks in English and one in Dutch of the CELF (Semel et al., 2003) were used. The results show that English vocabulary is a strong predictor of English grammar skills, and that the Dutch vocabulary skills are weaker predictors of English grammar skills. Moreover, Dutch grammar skills predict English grammar skills for one of the grammar tasks. These results are discussed vis-à-vis hypotheses about cross-domain transfer and cross-linguistic transfer (Blom et al., 2012; Cummins, 1979; Ganschow & Sparks, 1991; Paradis, 2011; Sparks, 1995).