This study examined the hypothesis that assisted writing during reading improves reading comprehension. The hypothesis was derived from sociocognitive and constructivist theory and research and implemented in the form of a curricular intervention called Writing Intensive Reading Comprehension after its main feature of bringing together reading comprehension and writing. The study used instruction with interactive thinksheets to provide sustained, focused, and assisted writing experiences to help students in low-performing urban schools construct understandings of their reading. The study used a cluster randomized trial design. Results support the hypothesis that bringing together reading and writing through instruction using thinksheets produces levels of reading comprehension superior to traditional instruction. Hierarchical linear modeling analyses of pretest and posttest data from 1,062 fourth- and fifth-grade students in 50 classrooms and 10 schools (four 2-year experimental, three 1-year experimental, and three control schools) also revealed greater effects for special education and low-income students in the experimental group. The study also revealed uneven improvement among experimental classrooms with regard to students' grade level, length of exposure to the intervention, and teachers' fidelity of implementation. The results imply that the upper grade students with the longer treatment and higher fidelity of implementation benefited more from the intervention. The study extends theory and research on reading-writing connections and suggests additional research and practical applications using assisted writing to improve reading comprehension.