We conducted a lexical study on the word frequency and the text coverage of the 570 word families from Coxhead's Academic Word List (AWL) in medical research articles (RAs) based on a corpus of 50 medical RAs written in English with 190425 running words. By computer analysis, we found that the text coverage of the AWL words accounted for around 10.07% in English medical RAs, that 292 (51.2%) out of the 570 AWL word families were frequently used in English medical RAs and that the academic words used in English medical RAs distributed dispersedly throughout a whole RA, accounting on the average for around 10% text coverage with slight difference among the five sections of a medical RA (Abstract, Introduction, Materials and Methods, Results, and Discussion). From these findings, we conclude that: (a) academic vocabulary, with a high text coverage and dispersion throughout a medical RA, is an important set of word items in medical RAs; (b) the AWL, a list of academic vocabulary representing academic word families across a wide range of subject disciplines, is far from complete in representing the academic words frequently used in medical RAs; and (c) the different coverage of academic words in the different sections in a medical RA, together with the role each section is supposed to play in a medical RA, indicates that academic words to a great extent serve some rhetorical functions in academic texts, as seen in the medical RAs in our study.