This paper examines changes in the average mathematics achievement of students in the Baltimore City Public School System (BCPSS) from 1998 to 2003, comparing students in schools that implemented Direct Instruction with students in other schools. First-grade students who received Direct Instruction had significantly higher levels of achievement on the Comprehensive Test of Basic Skills (CTBS) subtests of mathematics computations and mathematics concepts and applications. Differences were greater when measures of schools' sociodemographic context were controlled and as Direct Instruction became fully implemented and incorporated within the schools. Among students who began first grade in the BCPSS and remained in the same schools five years later as fifth graders, those who had received Direct Instruction as first graders had significantly higher scores on the measure of mathematics concepts and applications than students in the other schools. (Contains 5 tables.)