Knowledge of English vocabulary contributes to the learner's proficiency of English as a foreign language, but how the learner's lexical knowledge behaves in the contribution. Researchers in mainland China have conducted studies of various kinds in order to find out how the learner's lexical knowledge correlates with his proficiency. This article reviews the empirical studies on correlations between lexical knowledge and English proficiency over the past two decades. The correlations concerned in this review refer mainly to the relationships between lexical knowledge and the overall English proficiency, between the breadth and depth of lexical knowledge and the skills of listening, reading and writing in English as a foreign language.