The objective of this research was to determine various motivational factors that influence high school students towards intended careers in engineering. This study utilized data from the High School Longitudinal Study of 2009, which surveyed over 24,000 students in 9th grade and again in 11th grade. We classified students into four categories of intentions: 1) Not STEM Intending, those who did not select a STEM occupation in either the 2009 or 2011 administrations; 2) Leavers, students who selected a STEM occupation in 2009 but not in 2011, 3) Newcomers, those who did not originally specify a STEM career in 2009, but who did in 2011; and 4) Stayers, those who chose STEM careers in both 2009 and 2011. Repeated measures analysis of variance modeled the extent to which the motivational variables, measured for both mathematics and science, were related to students' career intentions. Results show that occupational intentions change dramatically between 9th and 11th grades, and that the relationship between STEM intention and motivation is highly time-sensitive: Of the 6,788 STEM intending students in 2009 (29% of the total sample), only 3,560 remained STEM intending in 2011 (48% attrition).