The question whether postbaccalaureate preparation before matriculation in medical school contributes to medical students' performance was addressed by this study. A total of 610 (91%) of the students who entered Jefferson Medical College between 1985 and 1987 were the study sample. Fifty-eight of these students had taken nondegree undergraduate premedical courses and 15 had taken nondegree graduate courses. Fourteen students held graduate degrees and 60 students had some combination of the aforementioned types of postbaccalaureate preparations. The other 463 students had not taken postbaccalaureate courses. Grades received in medical school courses such as anatomy, biochemistry, mechanisms of disease, physiology, microbiology, pathology, and pharmacology, as well as total scores on Part I of the National Board of Medical Examiners examination were selected as performance variables. Statistical analyses showed that the students who had taken nondegree postbaccalaureate courses had lower undergraduate grade-point averages than those without such courses and received lower grades on some measures of performance in medical school. The students with such additional academic backgrounds were also older than the average medical student. When adjustments were made for undergraduate grade-point averages by applying analysis of covariance, the observed differences that favored the group without postbaccalaureate preparation either became nonsignificant or favored those with such preparation. The differences favoring those without postbaccalaureate preparation could be accounted for mostly by these students' higher undergraduate grade-point averages and younger ages. Implications for admission decisions with regard to the changing applicant pool are discussed.