The skill of sentence writing -- the composition of connected text into sentence types understandable to readers -- is foundational to written expression. Unfortunately, many students with learning disabilities -- particularly those with attention deficit hyperactivity disorder (ADHD) and speech and/or language impairment (S/LI) -- struggle to develop sentence writing and its multiple, related skills. Using a single-baseline design, we studied effects of a supplemental writing intervention on one elementary-aged student with comorbid ADHD and S/LI. The intervention entailed delivery of a writing fluency intervention with several antecedent- and consequence-based components: explicit instruction, fluency practice, a reinforcer survey, and contingent reinforcement. The participating student improved his accuracy and speed of sentence writing during intervention. Results are discussed within the context of text writing fluency and frameworks of academic interventions for students with ADHD and S/LI.