Research has been investigating the role of reading, as one source of input, in language learners' vocabulary development. The present study was designed to examine whether intermediate learners incidentally (a) acquire and (b) retain unknown vocabulary as a result of reading. The study further assessed (c) the effect of the text variable of exposure frequency. Learners were exposed to unfamiliar words either two, four, or six times during reading. Vocabulary acquisition and retention measured productive and receptive knowledge gain. Results indicated that only two encounters with unfamiliar words during reading significantly affected learners' vocabulary growth. Moreover, two or four exposure frequencies resulted in fairly similar word gain, but six exposures produced significantly more vocabulary knowledge. Retention measures showed mixed results: On productive vocabulary knowledge only half of the subjects displayed a significant rate of retention. On receptive knowledge all but one experimental group retained vocabulary over 4 weeks.