The purpose of the study was to estimate the impact of reading intervention on ratings of student attention over time. We used extant data from a longitudinal randomized study of a response-based reading intervention to fit a multiple-indicator, multilevel growth model. The sample at randomization was 54% male, 18% limited English proficient, 85% eligible for free or reduced-price lunch, 58% African American, and 32% Hispanic. Reading ability was measured by using the Woodcock-Johnson III Tests of Achievement. Attention was measured by using the Strengths and Weaknesses of ADHD Symptoms and Normal Behavior Scale. Findings indicate that intensive, response-based reading intervention over 3 years improved reading achievement and behavioral attention in middle school struggling readers, with treatment directly affecting reading, which in turn influenced attention. In the business-as-usual condition, there was no relation between improved reading and attention. The results are consistent with a correlated liabilities model of comorbidity. The results do not align with the inattention-as-cause hypothesis, which predicts that reading intervention should not affect attention. The findings do not support, but do not necessarily preclude, the phenocopy hypothesis. The results are especially pertinent for older students who may be inattentive partly because of years of struggling with reading.