Students with emotional and behavioral challenges often struggle with writing. Curriculum-based measurement (CBM) is an assessment used for a variety of purposes including screening, progress monitoring, and data-based individualization. However, little research has examined technical properties of writing CBM data and supported inferences. This study examined criterion validity of two CBM tasks with the Test of Written Language-4 and whether writing fluency was affected by writing prompt type. Thirty-eight students in 7th-12th grades with emotional and behavioral challenges completed a descriptive writing task and a narrative writing task. Writing samples were scored for the number of words written, words spelled correctly, correct word sequences, and correct minus incorrect word sequences. Across all scoring procedures, associations between the descriptive prompt and the Test of Written Language-4 (number of words written, r = 0.27 [-0.12, 0.58], words spelled correctly, r = 0.33 [-0.05, 0.62]; correct word sequences, r = 0.38 [01, 0.65]; and correct minus incorrect word sequences, r = 0.44 [0.08, 0.69] were stronger than associations between the narrative prompt and criterion measure.