Objective: The first objective of this study was to compare the written expression competence of young adults with attention deficit hyperactivity disorder (ADHD) with that of young adults without ADHD on three types of measures: indicators of the story's microstructure with regard to productivity and morphosyntax; indicators of the macrostructure (story grammar); and expressions of the mental states of the story's characters (internal state language). The second objective consisted of determining the relationships among the different narrative writing measures and the ADHD behavior ratings of inattention and hyperactivity obtained using the family observer report. Method: The sample was composed of 54 participants, aged from 18 to 24, divided equally into a group with a childhood clinical diagnosis of combined subtype ADHD and a control group without ADHD. Written composition was assessed using a narrative task based on a sequence of images. Results: As expected, the young adults with ADHD obtained significantly worse results than the control group on the majority of the parameters of the story's microstructure and macrostructure. Likewise, they included a significantly lower number of terms about the characters' mental states. A correlation was also observed between measures of narrative competence and core symptoms of ADHD, as well as between the use of words with an emotional content and estimations related to core symptoms of ADHD. Conclusion: The findings of this study point out that expressive writing should be assessed in individuals with ADHD as part of screening and comprehensive evaluation. More research is needed to design and implement effective interventions in this area. (Contains 6 tables.)