Children with ADHD suffer from impairments in working memory, and recent studies have documented significant gains in working memory (WM) in children diagnosed with ADHD after participating in a PC-based WM training program. Earlier studies have focused on unmedicated children, while a majority of Norwegian children diagnosed with ADHD take ADHD medication for the disorder. The main question addressed in this study is whether ADHD children on medication would also show significant improvements in WM after PC-based WM training. A second issue examined is whether the results favor one of two established, but diverging, non-unitary models of the construct working memory. The results indicate that ADHD children on medication can improve on neuropsychological measures of verbal and visuospatial short-term memory by training systematically on computerized working memory tasks; the same gains on more complex verbal and visuospatial WM tasks were not registered in the current study. Investigations into possible transfer effects of the short-term memory gains to math and reading abilities, and the long-term effects of the training on functioning at home and at school will be needed before any conclusions or recommendations can be made about the benefits of the training program.