Background: In a longitudinal study we tested Frith's causal hypothesis that children first gain orthographic knowledge through reading and then later, as a consequence, through spelling. Method: Children from Years 2 and 3 were tested three times over two years on their reading and spelling of pseudo-words which conformed to the conditional orthographic rule, the "final -e" or "split-digraph" rule. Results: Cross-lagged panel correlation analyses suggested that the children's success in reading split-digraph words was a causal determinant of their learning to use split-digraphs in spelling, in the 7- to 8-year period and, with one year-group but not with the other, in the 8- to 9-year period. In the 9- to 10-year period children's success in reading no longer seemed to affect their spelling. Conclusions: These results strongly support Frith's causal hypothesis about the development of orthographic knowledge.