The aim of this study was to contribute to the scarce amount of research on self-selected Spanish foreign language (FL) vocabulary learning strategies (VLS) by adult learners of Spanish in the United States and to investigate which type of learning strategies may result in higher vocabulary gains and why. This study investigated the relationships between the type of VLS university Spanish FL students at different levels of proficiency use, the amount of time they devote to the weekly study of Spanish outside the classroom, and their vocabulary size. In addition, the correlations between the VLS used by students with high and low vocabulary test scores and their vocabulary size were investigated. A total of 477 military cadets/students at the United States Air Force Academy enrolled in Spanish courses at the beginning, intermediate, and advanced language proficiency levels participated in this study. The data were analyzed through quantitative methods using two measuring instruments: a) a vocabulary learning questionnaire used to discover students’ VLS preferences, and b) a Spanish vocabulary tests used to estimate the participants’ Spanish vocabulary size. Analyses of the data suggest that a significant relationship exits between learning strategy use and vocabulary size among advanced, more experienced Spanish learners but not among beginning- or intermediate-level students. Findings suggest that novice or inexperienced Spanish FL learners may be ineffective at the management of their own vocabulary learning. Different patterns in VLS use were also identified between advanced students with high and low vocabulary test scores. Those with higher vocabulary test scores use significantly more social and metacognitive learning strategies, while those with lower vocabulary test scores resort to memorization and other less-cognitively-demanding strategies for learning Spanish vocabulary. Pedagogical implications and limitations are addressed. / text