The main goal of this study is to report on the number of words (vocabulary load) native and non-native readers of medical textbooks written in English need to know in order to be able to meet the lexical demands of this type of subject-specific (medical) texts. For estimating the vocabulary load of medical textbooks, a corpus comparison approach and some existing word lists, popular in ESP and EAP, were used. The present investigation aims to answer the following questions: (1) How many words are needed beyond the General Service List (GSL; West, 1953), the Academic Word List (AWL; Coxhead, 2000), and the EAP Science List (Coxhead and Hirsh, 2007) to achieve a good lexical text coverage? and (2) What is the vocabulary load of medical textbooks written in English? The implementation of this corpus comparison approach consisted of: (1) making a written medical corpus of 5.4 million tokens, (2) compiling a general written corpus of the same size (5.4 million tokens), (3) running both corpora (i.e., the medical and general) through some existing word lists (i.e., the GSL, the AWL, and the EAP Science List), and (4) creating new subject-specific (medical) word lists beyond the existing word lists used. The system for identifying medical words was based on Chung and Nation's (2003) criteria for classifying specialised vocabulary. The results of this investigation showed that there is a large number of subject-specific (medical) words in medical textbooks. For both native and non-native speakers of English training to be health professionals, this figure represents an enormous amount of vocabulary learning. This paper concludes by considering the value of creating specialised medical word lists for research, teaching and testing purposes.