The current study aimed to examine students’ use of morphologically complex words in writing in relation to performance on other language tasks. In this study, language-learning data were collected on 490 students in fifth-grade classrooms. Measures included morphologically complex word (MCW) use in expository writing, decomposition and nonword derivation tasks, and general vocabulary and reading comprehension. There were significant small to moderate positive correlations between written MCW use and performance on decomposition and nonword MK tasks for students without developmental language disorders. For students with a developmental language-learning disorder (DLD), written MCW use was significantly correlated with decomposition skills. The measures of interest accounted for 18.34% of the variance in student MCW use, with the decomposition task as the only variable contributing unique variance.