Abstract: The paper examines the effects of High Dosage Math tutoring in three Dutch primary schools in low-income areas of Rotterdam. A randomly selected treatment group of 5th graders received high dosage math tutoring (2 students, 1 tutor), for four days/week, one hour/day, for the entire school year. The tutoring took place during regular class hours, and replaced some general math instruction as well as other courses. (Control group therefore received a “light treatment” by having small group instruction for four hours a week.) We find effects on math achievement for treated students after 6 months and after 10 months, with no measurable adverse effects on reading comprehension. Effects range between 0.2SD and 0.4SD, where 1SD is about equivalent to the difference in achievement between a 4th and a 5th grader.