Working memory contributes to many higher-order cognitive processes and predicts general cognitive skills. It is therefore important to know if its functions are trainable. In this study we investigated the malleability of working memory processes in schoolchildren whose cognitive functions are still developing. We also analyzed transfer effects to both general and specific intellectual skills. To address these issues, we examined the effectiveness of working memory training (10 training sessions) in terms of practice effects (trained tasks), near-transfer effects (working memory capacity), and far-transfer effects (psychometric intelligence). Sixty-nine children aged 8–10 participated in the study. The experimental group (42 children) participated in working memory training that intensely engaged the updating function of working memory. The training tasks, implemented as computer games, were based on the n-back and keep track paradigms. There was also an active control group (27 children). The results suggest that the experimental group improved their working memory capacity, as measured with both trained and untrained tasks. Regarding intelligence, far-transfer effects were weak and may be attributed to mere repetition of measurements. Moreover, whereas improvement in the training tasks could be observed after 15 months, the far-transfer effects disappeared in the delayed assessment.