The majority of research to date on pre-task planning has investigated the impact of planning time on L2 learners' oral production, and has generally reported its positive effects on their task performance. However, little research on planning has been conducted in writing contexts, and there is no firm evidence to demonstrate that pre-task planning promotes L2 learners' written production in the ways that many researchers have reported for L2 speaking contexts. In this paper, I have explored whether and how concept mapping as a form of pre-task planning could benefit the writing performance of three Japanese ESL learners. I analysed four compositions from each of the learners, written with and without concept mapping; using measures of accuracy, complexity, fluency and [Hamp-Lyons, L., 1991. Reconstructing "Academic writing proficiency". In: Hamp-Lyons, L. (Ed.), "Assessing second language writing in academic contexts." Ablex, Norwood, NJ, pp. 127-153.] holistic measures of global quality, communicative quality, organisation, argumentation, linguistic accuracy, and linguistic appropriacy. I also examined through a questionnaire, retrospective interview, and logs, the learners' applications of the strategy in their writing processes. Pre-task planning was associated positively with the overall measures of the learners' written production during in-class compositions, with the exception of accuracy. Moreover, each learner made unique applications of the concept mapping strategy in their writing processes, suggesting that concept mapping may help ESL learners improve their composing but in ways unique to individual experience, motivation, and task conditions. (Contains 4 figures and 4 tables.)