Background: Science internships have been suggested as a powerful way to engage high school students in conducting authentic science inquiry. However, despite the recognized significance of high school science internships, little research is done to examine how these experiences affect high school students' career choices. Purpose: Our study drew on the theoretical framework of social cognitive career theory to examine how a 7-month science internship might shape high school students' career choices. Method: 88 students were interviewed 6-8 months after their internship graduation. Findings: The analysis suggests that the science internships altered more than 90% of the participating students' career choices by either enhancing, expanding, narrowing down, or even replacing their original career choices. Students reported that the science internships boosted their self-efficacy through their first-hand mastery of authentic STEM practices, by directly observing scientists' STEM performance, by hearing scientists' opinions on students' capabilities and potential in STEM, and by the impact of the students' own physiological and affective states on the STEM practices. Implications: These findings help educators better understand how a unique learning environment like science internship may influence high school students' career choices; they have important implications for internship design, career counseling, and education policy.