The main aim of this study was to investigate the effect of a tier three intervention, response-to-intervention design, on children with low reading ability in grade three. Twenty-eight children (12 females and 16 males) participated in this study. The participants were given out a battery of reading tests including decoding and reading comprehension tests, and in total, the children received 20 reading intervention sessions in two waves, during 4 weeks. The results showed substantial gains with large effect sizes (d 0.78-2.95) on all the reading tests after the intervention period. A short, intensive and individualised intervention has a substantially positive effect on children's reading ability. For a majority of the children, the increased ability sustains even 4 years after the end of the interventions. However, as boys seem to have the greatest problem to sustain their increased ability, the authors claim that it is important to continue the intervention even after the research interventions have ended.