This study examined the impact of class size reduction (CSR) on achievement among 3rd, 4th, and 5th grade students with different numbers of years of participation in CSR. Single-year matched gains were calculated for the 1997-1998 and 1999-2000 school years. Student progress was assessed using scores from the SAT/9 reading, mathematics, and language achievement tests for those school years. Researchers compared 3rd graders with 3 years versus 1 year of CSR; 4th graders with 2 years versus 0 years of CSR; and 5th graders with 1 year versus 0 years of CSR. Overall, 3rd graders with 3 years of CSR had larger gains in reading and language than those with 1 year. In math, 3rd graders with 3 years of CSR out-gained those with 1 year. Fourth graders with 2 years of CSR scored significantly larger gains in reading than 4th graders with 0 years and exhibited somewhat larger gains in language than students with 0 years. In mathematics, there were losses for both groups. Fifth graders with 1 year of CSR showed lower gains in reading and larger gains in math than those with 0 years. CSR more significantly impacted the reading and language scores of 3rd grade English language learners with 3 years versus 1 year of CSR. (Contains 22 references.) (SM)