The purposes of this study are (1) to longitudinally measure the receptive vocabulary size (VS) of Japanese EFL senior high school learners; (2) to investigate how the learners develop their VS; and (3) to describe the longitudinal developmental patterns of VS of each learner during their three years of high school. Kasahara's (2005) VS tests, a modified version of Mochizuki's (1998) test were administered to 196 learners three times while they were in high school. Descriptive statistics were calculated and longitudinal comparisons were made, using the data of the identical subjects. One-way ANOVAs and post hoc Bonferroni tests confirmed that the general course learners made progress throughout their time at school. As for the commercial course learners, they made progress during their first and second years. However, there was no statistically significant difference in average scores between the second and third years. The statistics and the line graphs of longitudinally-measured VS revealed that in the commercial course, 14.7% and 42.7% of the learners made little and or negative progress between their first and second testing and their second and third testing respectively. The study results suggest the three main factors explaining this stagnation of the commercial course learners: school curriculum; motivation; and future career path. Further research should be conducted with learners whose academic levels vary widely to investigate this learning plateau.