Working Memory (WM) has a central role in learning. It is suggested to be malleable and is considered necessary for several aspects of mathematical functioning. This study investigated whether work with an interactive computerised working memory training programme at school could affect the mathematical performance of young children. Fifty-seven children with attention deficits participated in an intervention programme. The treatment group trained daily, for 30-40 min. at school for five weeks, while the control group did not get any extra training. Looking at the group as a whole, mathematical performance improved in the treatment group compared with the control group directly following the five weeks of training (Time 2), but the results of the second post-test (Time 3, approximately seven months later) were no longer significant. Since there was only a small number of girls, the results were analysed for boys only. The boys had improved their mathematical results in both post-tests. WM-measures improved at Time 2 and 3 relative to Time 1 (pre-test) for the whole group, and for boys. Differences in training scores were related to differences in the non-verbal WM-measure Span board back. The results indicate that boys aged 9 to 12 with special needs may benefit, over time, from WM training, as shown in the enhanced results in mathematics following WM training. However, as the intervention and control groups were not randomised, the results cannot be generalised; the results must be considered with caution.