A study examined the effectiveness of English dictionaries in assisting in error correction by beginning learners of English as a second language. Lexical errors made in examinations were collected and coded by type, and the usefulness of the "Longman Active Study Dictionary", designed for ESL learners, in helping to correct the errors was analyzed. The results suggest systematic differences in error types made by students of different language backgrounds. It is proposed that (1) analysis of errors made by language learners can be useful to dictionary writers in increasing the materials' effectiveness and (2) a particular dictionary can vary in effectiveness among different target language groups. Further study is both recommended and planned. (MSE)