This presentation reports on a study that compares the extent to which vocabulary size, lexical diversity, and lexical sophistication contribute to academic writing proficiency. Results suggest that lexical diversity has a greater impact on writing score over vocabulary size and lexical sophistication. Implications for practice and further analysis are discussed. Presented November 15, 2014 at the 2014 Symposium on Second Language Writing at Arizona State University in Tempe, Arizona, USA.