It is well established that extensive reading promotes the incidental learning of L1 and L2 receptive vocabulary; however, little is known about its effectiveness on productive gains in vocabulary knowledge. This paper investigates the extent to which extensive reading combined with writing tasks promotes productive vocabulary growth of Japanese university students. To examine changes in vocabulary size, three tests were administered: the Vocabulary Levels Test (Nation, 1990), the Productive Vocabulary Levels Test (Laufer & Nation, 1999), and the VocabProfile (Cobb, 2010). Contrary to expectations, no significant increase was shown in the amount of productive vocabulary size. However, the data suggested that one of the benefits of doing extensive reading with writing tasks is to retain receptive and productive vocabulary knowledge. (Contains 9 tables and 1 note.)