Although block scheduling has become increasingly popular in the past decade, only a few researchers have investigated its effect on academic achievement. Therefore, this study was conducted to determine the effects of block scheduling on academic achievement between 115 high school students who received instruction via a 4x4 block schedule and 146 students who received instruction via a traditional schedule. A series of independent t-tests, utilizing the Bonferroni adjustment, was conducted to compare grade point averages and scores on the Georgia High School Graduation Test (GHSGT) between the two groups. Findings revealed no statistically significant difference in grade point averages or in scores on the Writing portion of the GHSGT between the two groups. However, statistically significant differences were found for Language Arts (Cohen's d = 0.34, moderate), Mathematics (d = 0.52, large), Social Studies (d = 0.51, large), and Science (d = 0.46, large) scores. For each of the statistically significant differences, students who received instruction via a traditional schedule received the higher GHSGT scores. (Contains 1 table.)