This study investigated the long-term evolution of lexical diversity of adult L2 learners of French and Spanish (n = 33) over a five-year period. A longitudinal learner corpus was collected that includes oral (semi-structured interview, picture-based narrative) and written (argumentative essay) data. Three data collection waves are investigated in the current study: (a) before learners studied abroad as part of their university degree program, (b) at the end of their nine-month stay abroad, and (c) three years post-instruction. In addition to examining the potential outcomes of attrition, maintenance, or development, this study explores the extent to which variables such as peak attainment and language exposure/use predict changes in lexical diversity three years post-instruction. Results of ANOVA analyses indicate continued improvement in lexical diversity post-instruction for oral but not written tasks. Regression analyses indicate that both peak attainment in lexical diversity and language exposure/use contribute significantly to predicting gains in lexical diversity post-instruction on both oral tasks, with peak attainment explaining more of the variance than language exposure. The findings help us better understand the variables that influence the long-term evolution of foreign language proficiency after the conclusion of formal instruction.