This study aimed to model mathematics growth of students from Primary 3 to Secondary 3 from two approaches, namely, growth in content domains, and in cognitive domains. The sample consisted of 866 Hong Kong students who were followed longitudinally from Primary 3 to Secondary 3. The data were originally collected by the Education Bureau of Hong Kong. Plausible values of achievements at Primary 3, Primary 6, and Secondary 3 in three cognitive domains (Knowing, Applying, and Reasoning) were obtained using between-item multidimensional Rasch partial credit modelling. A modified autoregressive cross-lagged design was used to predict later achievement in the cognitive domains by prior achievements in these domains. Analysis was repeated for growth in content domains (Number, Shape and Space, Measures, Data Handling, and Algebra). Analysis showed that students' later mathematics achievement was strongly predicted by their previous achievement in the Knowing and the Number domains.[Copyright © 2014 Education Bureau, HKSARG.]