The effectiveness of Medical College Admission Test (MCAT) scores in predicting success during the first phase of medical education is investigated. The process by which medical students are educated and evaluated, the nature and purpose of the MCAT, and the MCAT Interpretive Studies Program developed by the Association of American Medical Colleges to study validity issues are described. Certain patterns in this preliminary data stage are that: (1) when the criteria are medical school course grades, linear composites of undergraduate college grades alone or MCAT scores alone appear to be equally predictive; (2) there is only a slight drop-off in prediction from first-year to second-year grades; (3) when the criterion is performance on National Board of Medical Examiners (NBME)-Part I, MCAT scores are substantially better predictors than undergraduate college grades; and (4) predictions of medical school course grade performance based on MCAT scores and undergraduate college grades are better than those based on either one alone; however, undergraduate college grades add little to the prediction of NBME performance by MCAT scores alone. The test scores appear to add unique information to that provided by undergraduate college grades alone, of a magnitude that is of practical significance to medical school admissions officers. (Author/PN)