Although much speculation has been devoted to concerns over the existence and degree of grade inflation at the high school level, there exists a lack of current empirical data documenting the extent, if it exists, of this phenomenon. This study was designed to investigate evidence of the existence, persistence, and degree of grade inflation by monitoring a large sample of schools over 5 years. Using only schools with at least 30 students taking the American College Test (ACT) resulted in a sample of 5,136 public schools. Decile ranks of student grade point averages and ACT results were compared. Results not only provide evidence supporting the grade inflation hypothesis, but also indicate that the phenomenon appears to be especially substantial at the higher end of the grade point scale. (Contains 5 tables and 13 references.) (Author/SLD)