The purpose of this study is to determine whether a vocabulary size test is a valid and reliable measure of longitudinal growth in vocabulary size (VS), and to investigate the relationship between growth in VS and use of vocabulary learning strategies (VLS). Kasahara's (2006) revised version of Mochizuki's (1998) Vocabulary Size Test (VST) was administered to 209 Japanese senior high school students at three time points during their course of study. In addition, a VLS questionnaire based on Schmitt's (1997) taxonomy was administered and used to investigate the relationship between vocabulary growth and patterns of strategy use. Cronbach alpha for the three administrations of the VST ranged from 0.79 to 0.85, indicating adequate reliability of the VST. A cluster analysis of strategy use yielded three distinct patterns: low, medium, and high frequency use of strategies. A two-way mixed ANOVA with year as the within-subjects factor and strategy cluster as the between-subjects factor found a significant mean effect for year and strategy use, but no effect for the interaction. The large effect size for year supports the claim that the revised VST is a valid measure of vocabulary size for Japanese high school students. The relationship between VS and strategy use confirms the potential of the revised VST for investigating theoretical and pedagogical factors affecting vocabulary growth.