Higher-functioning, school-age children with autism spectrum disorders (HFASD) or attention-deficit/hyperactivity disorder (ADHD) often experience writing difficulties. Prior research has not often investigated writing self-concept, self-efficacy, and text-type familiarity with these children, especially children with HFASD. This study looked at the responses from 132 school-age children with HFASD, ADHD, or typical development (TD) on a questionnaire focused on self-concept, text-type familiarity, writing dispositions, writing for school and non-school contexts, and writing attitudes. Results demonstrated diagnostic group differences in writing self-concept, text familiarity, and writing done for school and non-school contexts. These preliminary findings highlight the need to further understand the broader scope of writing development in school-age children with HFASD or ADHD to inform research and educational practices.