Many middle school students perform below grade-level standards in reading (National Center for Education Statistics, 2019), and recent observation studies demonstrate middle school teachers' limited use of reading comprehension practices within content area instruction (e.g., science and social studies; Greenleaf & Valencia, 2017). In this experimental pilot study, we aimed to boost middle schoolers' reading comprehension outcomes by providing schoolwide professional development (PD) on integrating reading comprehension practices within content instruction for English language arts, social studies, and science teachers. Six schools were matched into pairs and randomized to the schoolwide PD or a business-as-usual (BAU) condition. Content area teachers in schools assigned to the PD condition received distributed PD resources to support implementation, and coaching in one reading comprehension practice in the fall (i.e., get the gist) and one in the spring (i.e., asking and answering questions). Contrary to traditional PD, this PD was implemented across three content areas, was narrow in scope but long in duration (one practice per semester), focused on practices that could feasibly be integrated into content area instruction, and included ongoing coaching in content area teams. Students in schools assigned to the PD condition significantly outperformed those in the BaU condition on a measure of main idea generation (ES = 0.29) but not on measures of asking and answering questions (ES = 0.11) and general reading comprehension (ES = -0.09). Findings suggest promise for implementing schoolwide approaches embedded within content area instruction to improve reading comprehension performance for middle school students. [This paper was published in "Reading and Writing" v35 2022.]