The study investigated how well Swedish adolescents recognize the meaning of derived words in English and whether this knowledge is determined by learner proficiency, word frequency, or affix type. Participants were 88 school students in two proficiency groups: 39 advanced 12th graders and 49 intermediate 9th graders. Two tests of receptive vocabulary knowledge were administered. The first test had 80 basewords (e.g., "maintain") representing the 8,000 most frequent words. The second test had 60 words that were derived from the first test (e.g., "maintainable"). Results showed that participants' knowledge of basewords extended to knowledge of derived words quite well: for example, when the advanced learners knew a baseword, they knew its derived form in 91% of the cases. Other results were that word family frequency but not derived word frequency determined knowledge of derived words, and that little support was found for Bauer and Nation's (1993) hierarchy of affix difficulty. The main implication of the results is that the word family, which subsumes basewords and their related forms under word knowledge, is an appropriate unit of counting in L2 pedagogy and research for learners with extensive exposure to English and a Germanic first language.