This study investigated the long-term effects of 15 minutes of daily primary grade independent digital reading (IDR) on students’ reading comprehension. Subjects were tracked for three years (Grades 1-3), and evaluated for reading fluency and comprehension. Various techniques were used to rule out differential growth due to unobserved student or teacher characteristics. Treatment cohorts scored significantly higher on reading comprehension at the beginning of third grade than those in the control group (holding demographics and initial reading fluency equal). Consistent, successive IDR in Grades 1 and 2 significantly impacted reading comprehension in Grade 3. Both direct and indirect effects were identified. Those who were more fluent in reading tended to read more digitally, and those who read more digitally tended to become better at reading comprehension.