Growing concerns have been voiced about strategies employed in L2 reading in general and dealing with unknown words in particular. Among other strategies, lexical inferencing has received attention in the literature. However, more research is needed to further clarify how different levels of L2 reading proficiency may affect the readers' inferencing behavior with respect to the level of success they achieve as well as the types, combinations, and frequencies of knowledge sources they use when inferring meanings of unknown words. Taking up the legacy left by the pioneering works in lexical inferencing, the present study is an attempt to shed more light on the role of reading proficiency in the Iranian EFL learners' use of knowledge sources in second language lexical inferencing and their level of success in guessing the meanings of unfamiliar target words. To this end, a total of 86 Persian-speaking EFL learners inferred the meanings of unknown words in three authentic expository texts. The findings adduce the argument that differences in L2 reading proficiency have a profound impact on inferencing success. Further analysis of the protocols demonstrates that the contribution of the knowledge sources used by the participants to inferencing success is partial and restricted. (Contains 4 tables and 2 figures.)