Vocabulary is one of the building blocks of language and is a necessary component of learners' development. This paper aims to describe the development of the L2 lexicon from the first year of learning French as a foreign language at school to the last year of undergraduate studies at university by setting out what learners know and how this relates to academic results. This study focuses on vocabulary breadth. The definition for vocabulary breadth we use in this study relates to vocabulary size (i.e. the number of words learners know). Data were collected as part of the French Language Learner Oral Corpora (FLLOC) project using X_Lex--a yes/no vocabulary test in which students are asked to tick the words they know out of 100 words and 20 non-words. Analyses revealed that learners' lexicon size increased throughout the study period. Learning is linked to word frequency for all learners at all stages. However, no significant correlation was found between A-level exam grades (taken at the end of the final year of schooling, prior to university entry, at approximately age 18) and receptive vocabulary scores. The reasons for this are explored. The analysis also included comparisons with other recently collected data sets of the same nature (in other parts of the UK) suggesting possible regional differences. The paper contributes to the rapidly expanding body of literature on vocabulary acquisition. (Contains 5 tables and 5 figures.)