The use of curriculum-based measurement (CBM) for screening is well established, but there has been less research regarding the technical adequacy of written expression CBM (WE-CBM) for screening and the utility of this type of measure when used with students with diverse language backgrounds. The purpose of this study was to examine the validity and diagnostic accuracy of various WE-CBM indicators for 139 4th-grade students from diverse language backgrounds (89 native English-speaking students, 19 English language learners, and 31 Monitored students). We examined the validity of WE-CBM with the statewide writing achievement test and studied the diagnostic accuracy of WE-CBM for determining students at risk using receiver-operating characteristic curves. Results suggest that WE-CBM varies in validity and diagnostic accuracy across students and depending on the WE-CBM scoring indicator used. Additional research on the use of WE-CBM, particularly with diverse groups of students, is greatly needed.