Despite the growing body of research on writing assessment, little attention has been devoted to developing and validating measures for beginning writers. This study examined the technical adequacy of a Sentence Writing measure with 233 students in kindergarten and first grade. The reliability, validity, and sensitivity to growth were investigated for production scores (words written, correctly spelled words, and correct word sequences) and for a novel Qualitative Score. Most of the scores have promising alternate-form reliability and criterion-related validity with norm-referenced writing assessments. In addition, all scores were sensitive to bimonthly growth and differentiated between students in kindergarten and first grade. The results indicated that the Qualitative Writing Score may be a promising assessment for beginning writers.