The University of California faces the challenge of increasing the diversity of students graduating from its medical schools while also adhering to mandated restrictions on the use of race or ethnicity in the admissions process. Students from diverse backgrounds who gain admission as undergraduates to UC Berkeley and express an early interest in a medical career are an important potential source of medical students for the UC system. However previous data suggest that many of these undergraduate students lose interest in a medical career and never apply to medical school. We report on research that tracked the strength of students’ interest in a medical career from freshman matriculation through the end of sophomore year. Comparing students by self-described racial or ethnic group, we found a sharp decline in interest among all groups. We then interviewed a stratified sample of these students, asking what factors contributed to the observed decline in interest in premedical studies. Negative experience in science courses, principally chemistry, was the main factor contributing to students’ loss of interest. The adverse impact of chemistry courses was reported disproportionately by students from underrepresented minority groups, causing many of them to turn away from a possible medical career. If the UC system hopes to increase the diversity of its undergraduates who go on to medical school, it may need to reassess the structure and content of its undergraduate science curriculum, principally chemistry.