Class-size reduction (CSR) initiatives have demonstrated positive short- and long-term effects in elementary grades. Less is known about CSR influence on achievement in middle grades. Thus, we conducted a non-experimental, longitudinal, explanatory study of CSR influence on writing achievement of 3 independent cohorts of students (n = 123) in Grades 6 through 8. Analysis of variance was used to analyze results from the narrative writing section of the New Jersey Grade Eight Proficiency Assessment. The CSR had a statistically significant influence on the achievement of students who received CSR for 3 consecutive years (n = 38) and was practically significant (d = 0.61 and 0.79) compared to students who received CSR for one year (n = 42), and students who did not receive the treatment (n = 43), respectively. (Contains 4 tables.)