This study examined the accuracy of predictions of college grades in English, mathematics, social studies, and natural science courses, and the accuracy of predictions of overall freshman grade point average (GPA) based on American College Testing Program (ACT) assessment test scores and on high school course work and grade information from the ACT Assessment Course Grade Information Section (CGIS). Estimates of prediction accuracy (more than 160 institutions) were compared to those obtained using ACT scores and the four self-reported grades from the registration folder (TH index), ACT Assessment scores, and CGIS. Base-year prediction models were developed using student records from the 1986-87 and 1987-88 Prediction Research Services history files. These models were cross-validated using 1988-89 data from the same institutions. Separate models for juniors and seniors and the total group were developed. Results show that most ACT/CGIS models slightly increase prediction accuracy in some subject areas over that of the TH index. The model based on the four ACT scores and an average of 23 grades modestly improves prediction accuracy over that of the TH index for more than 50% of the institutions. Results support use of prediction models based on ACT scores and high school grades. Five appendices (one with five tables) provide supplemental information. Eight additional tables present study data, and there is a 39-item list of references. (Author/SLD)