Little is known about the reliability of college grades relative to how prominently they are used in educational research, and the results to date tend to be based on small sample studies or are decades old. This study uses two large databases (N > 800,000) from over 200 educational institutions spanning 13 years and finds that both first-year and overall college GPA can be expected to be highly reliable measures of academic performance, with reliability estimated at 0.86 for first-year GPA and 0.93 for overall GPA. Additionally, reliabilities vary moderately by academic discipline, and within-school grade intercorrelations are highly stable over time. These findings are consistent with a hierarchical structure of academic ability. Practical implications for decision making and measurement using GPA are discussed.