Frequent engagement in extramural English (EE) activities (i.e., English-language activities that students engage in outside of the classroom) has been shown to positively influence not only high school students’ vocabulary size and listening and reading comprehension, but also their oral proficiency (see, e.g., Sundqvist 2009; 2019; Sylvén & Sundqvist 2012). However, while previous studies have greatly contributed to our understanding of the relationship between EE and students’ receptive knowledge as measured through formal tests (e.g., of vocabulary, Sundqvist 2019), our understanding of the relationship between such activities and students’ production remains somewhat rudimentary (though see Sundqvist & Wikström 2015 and Olsson & Sylvén 2015). What is more, whereas vocabulary knowledge (both receptive and productive) features prominently in studies on EE, syntactic and broader lexical aspects have received very limited focus. As both syntactic and lexical complexity have been shown to be strongly correlated with writing quality (Casal & Lee 2019; Kyle & Crossley 2016), examining the relationship between EE activities and linguistic complexity would help us better understand the role that such activities play for students’ language development.