Research has established that academic intrinsic motivation, enjoyment of school learning without receipt of external rewards, significantly declines across childhood through adolescence. Math intrinsic motivation evidences the most severe decline compared with other subject areas. This study addresses this developmental decline in math intrinsic motivation, and also serves as a resource for applied researchers by providing exemplary illustrations of approaches to longitudinal modeling. Using a multivariate latent change model, the longitudinal relationship between academic intrinsic math motivation and math achievement among participants (n = 114) aged 9-17 years was examined to explain this motivational decline. On average, both math motivation and achievement decreased over time. This study reveals that math achievement is a significant contributor to the developmental decline in intrinsic math motivation from childhood through adolescence. In addition, academic intrinsic math motivation was found to be related to initial and later levels of mathematics achievement. These findings enhance understanding of developmental processes whereby early motivation and achievement are related to subsequent declines in mathematics. (Contains 4 figures, 2 tables and 1 footnote.)