The purpose of the study was to assess the volume of academic vocabulary in elementary grades disciplinary textbooks. Academic vocabulary was examined in a corpus of best-selling elementary grades textbooks in three disciplinary areas--science, mathematics, and social studies. Academic words in texts were determined through automated procedures involving statistical modeling. Four academic vocabulary variables were created: Total Academic Words; Discipline-Match Academic Words (science domain-specific academic words in science textbooks, and so on); High Challenge Total Academic Words; and High Challenge Discipline-Match Academic Words. Longitudinal multilevel Poisson regression was conducted for selected research issues. Main conclusions were: (a) The estimated overall elementary grades volume of academic vocabulary in disciplinary textbooks was relatively high. Summed across all grades and disciplines, 31% of all of the estimated unique word types in the textbooks were academic word types. By the end of elementary school, children who read or listened to disciplinary textbooks like the ones in the present study corpus would have been exposed to approximately one academic word type for every three unique word types encountered. (b) Moreover, approximately one or two of every four or five academic word types was estimated to be a word that would present challenge to typically developing and struggling students. (c) For all three disciplines, with minor exception, the estimated volume of newly-appearing academic words in a grade increased through the earliest grades, tended to peak in third or fourth grade, and then decelerated slightly thereafter.