This study examined the influence of a professional development project about an internet-based mathematics formative assessment tool and related pedagogies on primary teachers' instruction and student achievement. Teachers participated in 72 h of professional development during the year. Descriptive statistics and multivariate analyses of variance of the 300 teachers and data from 5300 students indicated that, in some of the participating districts, students whose teachers participated in professional development outperformed students in control classrooms. Multi-level analyses of student achievement indicated that teachers who used the formative assessment tool more had students who scored statistically significantly higher gain than students whose teachers did not use the tool as frequently. The findings call for subsequent studies that examine how teachers specifically use formative assessment data to make instructional decisions, and the influence of those decisions on student achievement.