This study contributes to the ongoing debate about how to count and organize words as part of efforts to optimize English as a foreign language (EFL) instruction. We examined the degree to which 150 low and high proficiency EFL learners’ meaning recall of base words (e.g., develop) overlapped with their knowledge of their derivational forms (e.g., developer, redevelop). Learners’ meaning recall of derivational forms was also explored in relation to the form’s word frequency and whether derivational forms presented with semantically contextualized support were more likely to be known. We found that Japanese learners’ knowledge of derivational forms was limited. If they knew a base word, they were likely to know its derivational form about 50% of the time. Also, the higher frequency derivational forms were more likely to be known. The provision of contextualized support, however, did not enhance learners’ capacity to recall the meaning of derivational forms.