Children with low performance in early numeracy are at risk of facing learning difficulties in mathematics, but few trials have examined how this can be ameliorated. A total of 120 first-grade children (M[subscript age] = 6.4 years) were randomly assigned to an intervention or a control condition. The 14-week intervention targeted early numeracy skills and was delivered in small groups three times a week. Immediately after the initial 8-week intervention phase, moderate and positive effects were found on early numeracy (d = 0.19), word problem solving (d = 0.41), and approximate number sense (d = 0.35). However, only the effects on word problems were significant, and all effects disappeared after the children undertook a second 6-week intervention phase. Overall, results indicate that (a) early numeracy skills are malleable in low-performing children, but (b) frequent and long-term interventions are needed for the positive effects to last.