Previous studies suggest that individuals with an Autism Spectrum Disorder (ASD) are more likely than other disability groups and the general population to gravitate toward science, technology, engineering, and mathematics (STEM) fields. However, the field knows little about which factors influenced the STEM pipeline between high school and postsecondary STEM major. This study analyzed data from the National Longitudinal Transition Study--2, a nationally representative sample of students with an ASD in special education in the United States. Findings suggest that students with an ASD who took more classes in advanced math in a general education setting were more likely to declare a STEM major after controlling for background characteristics and previous achievement level. Educational policy implications are discussed. Tables are appended. [This paper was published in "Focus on Autism and Other Developmental Disabilities" v32 n2 p83-92 Jun 2017 (EJ1141715).]