This article reports on a study that investigated the extent to which lexical frequency and lexical diversity contribute to writing proficiency scores on monolingual English-speaking writers' and advanced multilingual writers' academic compositions. The data consist of essays composed by 104 multilingual English learners enrolled in advanced second language writing courses at various intensive English programs and 68 monolingual English-speaking university students in a first-year composition course. Three independent raters evaluated the essays according to the TOEFL iBT independent writing rubric. Results from a binary logistic regression reveal that lexical diversity has a significantly greater impact on writing score than lexical frequency. Post hoc analyses show that lexical frequency has only a moderate relationship to lexical diversity, suggesting that diversity of mid-frequency vocabulary may be more important to writing proficiency than the use of more sophisticated terms that occur less frequently in natural language. Implications for practice suggest that it is not enough to simply teach vocabulary words in the second language composition classroom, but also to guide learners in how to employ these words in a varied manner in their writing.