This study explored the relationship between middle school students' mathematics- and science-related motivational dispositions and their attitudes toward STEM-related careers. To better understand these constructs, we regressed motivational variables from the Situated Expectancy-Value Framework (Eccles, J. S., & Wigfield, A. 2020. From expectancy-value theory to situated expectancy-value theory: A developmental, social cognitive, and sociocultural perspective on motivation. "Contemporary Educational Psychology," 61, 101859), on attitudes toward STEM careers by gender, race/ethnicity, and age. Findings showed that, among the five motivational disposition variables (i.e. self-efficacy, identity, enjoyment, utility value, and cost), utility value was the strongest predictor of STEM career attitudes and science-related motivational dispositions explained the variation in STEM career attitudes more than mathematics-related motivational dispositions. We also found changes in the strength of the relationship between students' motivational dispositions and STEM career attitudes by gender, race, and age. Finally, we reported an indirect effect of self-efficacy on STEM career attitudes mediated by identity, interest, utility value, and cost, a finding that may explain the perplexing results by prior research that contradicts some current theories of self-efficacy and career motivation.