The purpose of this research is to study the long-run impacts (i.e. on educational attainment) of a freshman math intervention called "double-dose algebra". The intervention was conducted in 2003 and 2004 within the Chicago Public Schools (CPS), a large, poor urban school district. In response to low passing rates in 9th grade algebra, CPS launched the double-dose algebra policy for students entering high school in the fall of 2003. Instead of reinstating the traditional remedial courses from previous years, CPS required enrollment in two periods of algebra coursework for all first-time 9th graders testing below the national median on the math portion of the 8th grade Iowa Tests of Basic Skills (ITBS). Such students enrolled for two math credits, a full-year regular algebra class plus a full-year algebra support class. The authors follow two cohorts of students from the Chicago Public Schools from their freshman high school fall (in 2003 and 2004) through the end of high school and the beginning of college. The main analytic sample contains upwards of 11,000 students, 95% of whom are poor and black or Hispanic. Double-dosing increased the proportion of students earning at least a B in freshman algebra by 9.4 percentage points, a more than 65% increase from a base of 13.8 percentage points. Though passing rates for freshman algebra increased by 4.7 percentage points, the increase is statistically insignificant. Double-dosed students were also no more likely to pass geometry. Results show that math instruction was most successful for students with average math skills but relatively low reading skills, and not successful for the average treated student. This highlights the importance of carefully targeting such interventions to students most likely to benefit from them. Tables and figures are appended.