Growing a large vocabulary is one of the most important tasks of language learners and yet research reports on low levels of lexical knowledge for English as a foreign language (EFL) learners. In EFL contexts, textbooks are a vital source of exposure to English and the words that are worth learning. The New Concept English textbook series is widely used in China by large numbers of learners in secondary schools and cram schools, and in preparation for university studies. This study finds that while learners will encounter mostly high-frequency vocabulary in these books, they will need to know between 3000 and 6000 word families to cope with these texts, and Book 4 requires 1000 more word families than Book 3. Science-related texts in the books contribute to the vocabulary load because they contain a large proportion of low-frequency words used only once and have a higher vocabulary load than humanities texts. There are opportunities for learning mid-frequency vocabulary in these textbooks. Limitations of current research and suggestions for future research are also presented.