Ease of processing of 3,969 English words for native speakers and Japanese learners was investigated using lexical decision and naming latencies taken from the English Lexicon Project (Balota et al. The English Lexicon Project: A web-based repository of descriptive and behavioral measures for 40,481 English words and nonwords, 2002) and accuracy of English word translation by Japanese university students (Takashima, H. "Eigo goi chishiki no keisei" ["The structure of English lexical knowledge of Japanese college students"], 2002). Correlations among these ease-of-processing values were all significant, suggesting substantial commonalities between native English speakers and Japanese learners. Regression analyses, however, showed that some factors differentially affect ease of processing for natives and Japanese. Comparison of the predicted and the observed values of translation accuracy revealed specific differences of lexical knowledge between native speakers and Japanese learners. Loanword effect on translation accuracy and translation errors similar to dyslexic/aphasic reading errors were observed, suggesting the possibility of insufficient orthographic/phonological activation and the possibility of the use of first language phonological representations. The implications of these results for the study of second/foreign language lexical processing are discussed.