Star Math (SM) is a popular computer adaptive test (CAT) schools use to screen students for academic risk. Despite its popularity, few independent investigations of its diagnostic accuracy have been conducted. We evaluated the diagnostic accuracy of SM based upon vendor provided cut-scores (25th and 40th percentiles nationally) in predicting proficiency on an end of year state test in a sample of highly achieving grade three (n = 210), four (n = 217), and five (n = 242) students. Specificity exceeded sensitivity across all grades and cut-scores. Acceptable levels of sensitivity and specificity were achieved in grade three and four but not grade five when using the 40th percentile.