In recent years, students' test scores have been used to evaluate teachers' performance. The assumption underlying this practice is that students' test performance reflects teachers' instruction. However, this assumption is generally not empirically tested. In this study, we examine the effect of teachers' instruction on test performance at the item level using a hierarchical differential item functioning approach. The items are from the U.S. TIMSS 2011 4th-grade math test. Specifically, we tested whether students who had received instruction on a given item performed significantly better on that item compared with students who had not received such instruction when their overall math ability was controlled for, whether with or without controlling for student-level and class-level covariates. This study provides preliminary findings regarding why some items show instructional sensitivity and sheds light on how to develop instructionally sensitive items. Implications and directions for further research are also discussed.