A medical reasoning aptitude test (MRAT) was designed to assess aptitude for clinical problem-solving in medical school applicants. The purpose of the study reported here was to determine whether the information provided by this test, when used in conjunction with college grade-point averages (GPAs) and scores on the Medical College Admission Test (MCAT), would improve the prediction of medical school performance. Specifically, the authors investigated the incremental predictive value of the MRAT relative to students' overall performance in medical school and, more specifically, to their knowledge and clinical reasoning during preclinical years and clinical performance during the clerkship year. Overall, it was found that, with the exception of the first year, the addition of MRAT scores to the GPAs and MCAT scores increased precision in identifying students who performed poorly or exceptionally well in the second year and the clinical clerkship year. This last finding is especially useful, since few other tests have provided that information. On the basis of the preliminary findings, the authors propose further use and validation of the MRAT.