The study evaluated the reliability and validity of the Writing Assessment Measure (WAM), developed to reflect the skills which children of different abilities are expected to achieve in written expression, as part of the National Curriculum guidelines in England and Wales. The focus was on its potential use in investigations of children's written narrative in order to inform and target related interventions. The study involved 97 children aged 7–11 from one urban primary school in England. Prompt 1 was administered to all the children in their classrooms together with a standardised written expression test. After three weeks, the same procedure was followed and Prompt 2 was administered. Statistical analyses of the reliability and validity of the instrument showed that it is consistent over time and can be scored reliably by different raters. Content validity of the instrument was demonstrated through inspection of item total correlations which were all significant. Analyses for concurrent validity showed that the instrument correlates significantly with the Wechsler Written Expressive Language sub-test. Significant differences between children of different age and writing skill were also found. The findings indicate that the instrument has potential utility to professionals assessing children's writing.