The effect of block scheduling on high school student achievement in mathematics and reading was investigated in this study through the use of an ex postfacto, longitudinal research design. Specifically, student scores from 9thand 11th-grade standardized tests were matched and sorted by junior high and high school attended. Outcome measures consisted of Levels tests and ACT exams in mathematics and reading. Statistical analyses of student gain scores included main effects of scheduling type, gender, and ethnicity as well as interaction effects for these independent variables. Results indicate that students in 4 X 4 block scheduling had greater gain scores in reading and mathematics than did students in both traditional scheduling and A/B block scheduling.