Purpose: The aims of the current project were twofold: (a) to describe the use of academic words in written language samples by fifth-grade students and (b) to examine the predictive relation between academic word use in academic writing and reading comprehension. Method: Investigators utilized written expository responses of 1,128 students in fifth grade who differed in English proficiency and language ability. The sample included 214 students who were English learners (ELs) and 144 students with identified language learning disabilities (LLD). Group differences in the use of academic words from the Coxhead word list were examined. Results: ELs and students with LLD used academic words less frequently than their peers and demonstrated less variety in their academic word use. There was a significant relation between students' use of academic words and reading comprehension. Academic word use accounted for 16% of the variance in reading comprehension, which was not significantly different for ELs or students with LLD. The relation was moderated by economic advantage, with the strength of the relation being lower for students who were eligible for free/reduced lunch. Conclusions: Findings support the need for additional research on ways to improve academic vocabulary skills to minimize achievement gaps. The relation between academic word use and reading comprehension warrants further consideration.