It is known that computer games are motivating for children, but there is limited direct evidence of their effects on classroom learning. The aim of this exploratory study was to investigate the effects of a commercial off-the-shelf computer game on children's mental computation skills and on aspects of self-perceptions. A pre-post design was employed. The participants were 71 primary school children (10-11 years old) from three classes. In School 1, a class of 21 children used a games console for 20 minutes each day, running a "brain training" game. Two comparison groups were used. In School 2, 31 children used "Brain Gym" techniques in their class over the treatment period. In school three, a class of 19 children acted as no-treatment controls. The treatment period was 10 weeks. Significant pre-post gains were found in the games console group for both accuracy and speed of calculations, while results for the two comparison groups were mixed. The games console group showed significant gains in global self-esteem, but not in other aspects of self-concept. The comparison groups showed no significant gains in any area of self-perceptions. There is a need now for upscaling to investigate generalisability.