This longitudinal study investigates the development of writing proficiency in English as a foreign language (EFL), in contrast to the development of first language (L1) writing proficiency in Dutch L1, in a sample of almost 400 secondary school students in the Netherlands. Students performed several writing tasks in both languages in three consecutive years. Furthermore, data were collected about students' metacognitive and linguistic knowledge (grammar, vocabulary, and spelling) and their fluency in lexical retrieval and sentence building (reaction times). Analyses, using structural equation modeling, show that EFL writing was more strongly correlated to linguistic knowledge and linguistic fluency than L1 writing was and that, over the course of the two years investigated, students' EFL writing proficiency improved to a greater extent than did their L1 writing proficiency. Furthermore, through the modeling of L1 and EFL writing proficiency, a strong relation between the two constructs could be established, with metacognitive knowledge and general fluency mediating this relation. This finding is paralleled by the study of Van Gelderen, Schoonen, Stoel, De Glopper, and Hulstijn (2007) showing a strong relationship between L1 and EFL reading proficiency. Taken together, the findings of these studies call for the inclusion of the constructs of L1 proficiency, linguistic fluency (speed of processing of lexical and grammatical information), and language-general metacognition in theories of the acquisition of L2 proficiency.