In this study we investigated whether teachers' pedagogical and content knowledge of writing would increase as an outcome of teachers taking a research lens to their practice to raise students' writing achievement. Using student achievement data as a baseline, teachers examined and refined their practice using an inquiry process. The study took place over a two-year period and involved over 20 teachers from six low socio-economic urban primary schools in Auckland, New Zealand. Literacy leaders in the schools and four university researchers also took part in the inquiry project. Data collected from teachers' records, researchers' field notes, and transcripts from focus groups of teachers and literacy leaders indicated enhanced pedagogical and content knowledge of writing, as well as marked gains for students on a standardized test of writing. This study contributes to research demonstrating that, through researching their own practice and teaching targeted to students' strengths and needs, achievement in writing can be raised. (Contains 2 tables, 1 figure and 1 footnote.)