This study investigated the stability of students' grades in high school courses during the academic year. Records of over 8,000 high school students from five large southeastern high schools were analyzed to determine the relationship between the first achievement grade students were assigned during an academic term and their final course grades. Results showed that grades were remarkably stable, with correlations between first achievement grades and final course grades generally ranging between +0.4 and +0.7. Correlations were typically larger for 9th- and 10th-grade courses than 11th- and 12th-grade courses. Multiple regression results revealed that differences among departments (subject areas) were statistically significant, but differences among schools were not. Differences related to student gender, grade level, ethnicity, and poverty level also were statistically significant but modest in magnitude. The implications of such stability and its probable influence on student motivation are discussed. (Contains 6 tables.)