Postsecondary education plays a critical role in building a strong workforce in Science, Technology, Engineering, and Mathematics (STEM) fields. The U.S. postsecondary education system, however, frequently loses many potential STEM graduates through attrition. An increasing portion of STEM leavers are top performers who might have made valuable additions to the STEM workforce had they stayed in STEM fields. Using data from the 2004/09 Beginning Postsecondary Students Longitudinal Study (BPS:04/09), this study tracks a cohort of U.S. beginning bachelor's degree students over 6 years, providing a close look at STEM attrition among a group of high-performing college students. Capitalizing on the transcript data collected through BPS:04/09, this study also examines STEM coursetaking, detailing how participation and performance in undergraduate STEM coursework are associated with students' departure from STEM fields. The study finds that about a quarter of high-performing beginning bachelor's degree students entered STEM fields (i.e., declared a STEM major) during their enrollment between 2003 and 2009, and about a third of these entrants had left STEM fields by spring 2009. The results of multinomial probit regression analysis indicate that students' intensity of STEM coursework in the first year and their performance in STEM courses may have played an important role in their decisions to switch majors out of STEM fields.