This study is an attempt to clarify the incremental and multidimensional nature of foreign language vocabulary development and its relation to the participants' reading and writing performances and general language ability of English as a foreign language (EFL). With this principle aim, the current study investigated the relationship between receptive and productive vocabulary knowledge, the relationship between receptive vocabulary knowledge and reading performance and the relationship between productive vocabulary knowledge and writing performance using the participants' scores on vocabulary knowledge tests, a reading exam and a writing exam. Additionally, the lexical level of the essays written by the participants and its relation to their productive vocabulary knowledge and the impact of both receptive and productive vocabulary knowledge on the participants' general language ability of EFL were also examined. 175 students studying in an intensive language program participated in the study. The results revealed that the students' receptive vocabulary knowledge was larger than their productive vocabulary knowledge. It was also found that the contribution of vocabulary knowledge to the foreign language performances of reading, writing and proficiency was significant. Moreover, according to the Lexical Frequency Profile results, the lexical level of the student essays and the students' productive vocabulary knowledge were significantly related.