This study investigated the effects of corrective feedback (CF) during in-class computer-mediated collaborative writing on grammatical accuracy in a new piece of individual writing. Forty-eight ESL students at an American university worked on two computer-mediated animation description tasks in pairs. The experimental group received indirect CF on English indefinite and definite articles from the researcher during the tasks, while the comparison group worked on the same tasks without CF. Each computer screen was recorded during the treatment, so that the number of revision contributions from each individual learner could be identified. L2 development was measured by a pretest, posttest, and delayed posttest, where the students worked on an animation-description task without a partner. A repeated-measures ANOVA indicated a significant relationship between the presence of CF and accuracy improvement over time. Furthermore, multiple regression analyses suggested a significant relationship between the number of learners' revision contributions and the delayed posttest scores when the pretest scores held constant. That is, individual learners' long-term L2 development varied depending on the extent to which they contributed to the revision. These findings demonstrate the importance of tracking individuals' contributions while calling for more detailed collection of data on actual revisions and the distribution of revision work within pairs or groups.