This study examines the impact on student achievement of high-dosage reading tutoring for middle school students in New York City public schools, using a school-level randomized field experiment. Across three years, schools offered at least 130 hours of 4-on-1 tutoring based on a guided reading model. We demonstrate that, at the mean, tutoring has a positive and significant effect on school attendance, a positive, but insignificant, effect on English Language Arts (ELA) state test scores and no effect on math state test scores. There is important heterogeneity by race. For black students, our treatment increased attendance by 2.0 percentage points (control mean 92.4 percent) and ELA scores by 0.09 standard deviations per year – two times larger than the effect of KIPP Charter Middle Schools on reading achievement. We argue that the increased effectiveness of tutoring for black students is best explained by the average tutor characteristics at the schools they attend.