Various researchers in second language acquisition have argued for the effectiveness of immediate rather than delayed feedback. In writing, truly immediate feedback is impractical, but computer-assisted feedback provides a quick way of providing feedback that also reduces the teacher's workload. We explored the accuracy of feedback from Criterion®, a program developed by Educational Testing Service, and students' responses to it. Thirty-two students received feedback from Criterion on four essays throughout a semester, with 16 receiving the feedback immediately and 16 receiving it several days after writing their essays. Results indicated that 75% of the error codes were correct, but that Criterion missed many language errors. Students responded to the correct error codes 73% of the time and responded to more of the codes over the course of the semester, while the condition--delayed versus immediate--did not affect their response rates nor their accuracy on the first drafts. Although we cannot support claims that immediate feedback may be more helpful, we believe that, with proper training, Criterion can help students correct certain aspects of language.