Several studies have reported the benefits of peer reviews in English as a second language (ESL) and English as a foreign language (EFL) writing classrooms. However, there has been little empirical research on whether such peer reviews improve students' writing abilities. The current study investigated the effects of peer review on the development of writing abilities. An experimental group conducted peer reviews in addition to writing practice with teacher feedback, and a control group only practiced writing and received teacher feedback. The study was conducted over a 12-week period in a high school EFL classroom. At the beginning and end of the research, pre- and post-essay tests were administered to determine whether the peer reviews had had a positive effect on the development of student writing abilities. In addition, the researcher conducted a delayed posttest to examine whether the improvements in writing skills were retained. She also used questionnaires and interviews at the end of the research to examine the relationship between the test results and the peer review feedback. The findings show that peer reviews contributed to an improvement in student writing abilities, which included improvements that were retained after the peer review practice ended.