This paper reports on the longitudinal results over 12 months of 342 Grade 1 children from 57 schools in Sydney, Australia, who participated in the "Extending Mathematical Understanding" (EMU) Intervention Program. Their vulnerability in four whole number domains is described, and the extent to which this changed after 12 months. Overall the EMU students made accelerated progress when compared with all Grade 1 students (N = 3277) and the number of domains for which they were vulnerable decreased when assessed 12 months later. However, 75% of the children who received this supplementary support remained vulnerable in at least one number domain the following year.