National assessment data indicate that the large majority of students in America perform below expected proficiency levels in the area of writing. Given the importance of writing skills, this is a significant problem. Curriculum-based measurement, when used for progress monitoring and intervention planning, has been shown to lead to improved academic achievement. However, researchers have not yet been able to establish the validity of curriculum-based measures of writing (CBM-W). This study examined the structural validity of CBM-W using exploratory factor analysis. The participants for this study were 253 third, 154 seventh, and 154 tenth grade students. Each participant completed a 3-minute writing sample in response to a narrative prompt. The writing samples were scored for fifteen different CBM-W indices. Separate analyses were conducted for each grade level to examine differences in the CBM-W construct across grade levels. Due to extreme multicollinearity, principal components analysis rather than common factor analysis was used to examine the structure of writing as measured by CBM-W indices. The overall structure of CBM-W indices was found to remain stable across grade levels. In all cases a three-component solution was supported, with the components being labeled production, accuracy, and sentence complexity. Limitations of the study and implications for progress monitoring with CBM-W are discussed, including the recommendation for a combination of variables that may provide more reliable and valid measurement of the writing construct.