Background: Until now, working memory training has not reached sufficient evidence as effective treatment for ADHD core symptoms in children with ADHD; for young children with ADHD, no studies are available. To this end, a triple-blind, randomized, placebo-controlled study was designed to assess the efficacy of Cogmed Working Memory Training (CWMT) in young children with ADHD. Methods: Fifty-one children (5-7 years) with a "DSM-IV-TR" diagnosis of ADHD (without current psychotropic medication) were randomly assigned to the active (adaptive) or placebo (nonadaptive) training condition for 25 sessions during 5 weeks. The compliance criterion (>20 sessions) was met for 47 children. The primary outcome measure concerned the core behavioural symptoms of ADHD, measured with the ADHD Rating Scale IV (ADHD-RS). Secondary outcome measures were neurocognitive functioning, daily executive functioning, and global clinical functioning. The influence of the increase in difficulty level (Index-Improvement) for the treatment group was also analysed. Clinical trial registration information--"Working Memory Training in Young ADHD Children"; www.clinicaltrials.gov; NCT00819611. Results: A significant improvement in favour of the active condition was found on a verbal working memory task (p = 0.041; adapted Digit Span WISC-III, backward condition). However, it did not survive correction for multiple testing. No significant treatment effect on any of the primary or other secondary outcome measurements was found. The Index-Improvement significantly contributed to ADHD-RS and the Behavior Rating Inventory of Executive Function, both rated by the teacher, but revealed no significant group difference. Conclusions: This study failed to find robust evidence for benefits of CMWT over the placebo training on behavioural symptoms, neurocognitive, daily executive, and global clinical functioning in young children with ADHD.