This study investigates the effects of cognacy on vocabulary learning. The research expands on earlier designs by measuring learning of English-Japanese cognates with both decontextualized and contextualized tests, scoring responses at two levels of sensitivity, and examining learning in a more ecologically valid setting. The results indicated that Japanese learners could successfully recall the L2 forms of more cognates than noncognates, supporting earlier findings. However, when scoring was sensitive to partial knowledge of written form, the results indicated that greater knowledge of noncognates was gained. Because there was greater potential for learning noncognates due to the higher pretest scores for cognates, relative gains were also examined. The relative gains were greater for cognates than noncognates on a form recall test. The results of a cloze test contrasted with those of the form recall test. Gains were significantly larger for noncognates than cognates immediately after the treatment although no statistically significant difference existed 1 week after learning. Taken together, the research indicates that although the L2 forms of cognates may be more easily learned, it may be more challenging for second language learners to use cognates than noncognates, at least shortly after learning.