We examined written expression performance in a sample of adolescents with ADHD and subthreshold ADHD using two different strategies: examining performance on standardized measures of written expression and using other indicators of written expression developed in this study. We examined associations between standardized measures of written expression, cognitive processing measures (working memory, processing speed, language, fine motor ability, and reading efficiency) and behavioral ratings of ADHD by parents and teachers. We also developed a coding scheme for a writing sample to measure productivity and the ratio of self-corrections to errors. The results indicated that written expression performance was most consistently associated with cognitive processing measures and not behavioral ratings of ADHD, based on correlational and simultaneous regression analyses. These results were consistent in the analyses with both the standardized measures and the coding scheme measures of written expression. Findings generally remained robust, regardless of whether participants who met criteria for a learning disability were included or excluded in the analyses. The current results suggest that written expression difficulties in adolescents with ADHD are attributable to processing difficulties that may be associated with ADHD, not to ADHD reported symptoms. Implications for assessment and intervention are discussed.