In paired-associate learning, there are two learning directions: L2 to L1 (L2 words as stimuli and L1 words as responses) and L1 to L2 (L1 words as stimuli and L2 words as responses). Results of previous studies that compared the effects of the two learning directions are not consistent. We speculated that the cause of this inconsistency may be L2 proficiency, as the strengths of the lexical links between L2 and L1 are different depending on the learner's L2 proficiency. This hypothesis was examined with 28 native speakers of Japanese learning English. Participants studied novel English words in the two learning directions. The results of posttests showed that for lower-proficiency learners, L2-to-L1 learning was more effective than L1-to-L2 learning, while for higher-proficiency learners, L1-to-L2 learning was more effective. The findings suggest that L2 proficiency influences the effects of learning direction on vocabulary learning.