BACKGROUND: Premedical students often elect advanced science courses whose content will reappear during preclinical courses. Are such “preludes” useful? METHOD: The study participants were the 176 first-year students entering the University of Iowa College of Medicine in 1992. Their grades in medical school courses in biochemistry, gross anatomy, histology, physiology, and microbiology were compared with their grades in similar premedical courses. RESULTS: The students who had taken a premedical prelude in advanced science performed no better than their classmates except in biochemistry, where the 118 students (67%) with prior biochemistry exposure had a significantly higher mean score (96.3 vs 87.6, p < .0001 using Student's t-test). A biochemistry prelude appeared to benefit all students, especially those from minorities underrepresented in medicine. In addition, among the 13 students who failed biochemistry in medical school, a number of them had low grades in organic chemistry and had not taken a premedical course in biochemistry. To test the replicability of the findings, an analysis was undertaken of the biochemistry performances of the 162 students who had entered in 1991, and again a significant difference was found between the students who had and those who had not taken a biochemistry prelude. CONCLUSION: A premedical biochemistry course, required by only two schools in 1995-96, appears advantageous, especially for students with weak academic preparation. Lack of such benefit from other preludes suggests that premedical students might better choose electives in arts and humanities to enhance their educational breadth.