Research on growth mindset, the belief that one's cognitive abilities are malleable and can be developed through dedication and practice, has received considerable media attention and influenced educational policy and practice. However, mindset theory and measurement have also drawn criticism. In the present paper, we add a cautionary note pertaining to the conceptualization and measurement of growth mindset. Through a critical reanalysis of a large-scale representative study of adolescents from the US (N = 15,362), we show that a growth (i.e., forward-keyed) and a fixed (i.e., reverse keyed) mindset item from a widely used scale are only moderately correlated (r = -0.31). Further, we demonstrate that the two items are very differently related with a range of educationally relevant criteria such as learning engagement and self-efficacy, and sociodemographic characteristics such as sex. This leads us to conclude that the growth and fixed mindset items are not mutually interchangeable (apart from keying) indicators of a unidimensional construct that has fixed and growth mindset at its opposing poles. Which items researchers choose to measure mindset (fixed, growth, or a blend thereof) may therefore have a significant impact on the findings they obtain. Our insights highlight the need for greater attention to the conceptual foundations and measurement of mindset in future studies.