Background: People with autism spectrum disorders (ASDs) experience executive function (EF) deficits. There is an urgent need for effective interventions, but in spite of the increasing research focus on computerized cognitive training, this has not been studied in ASD. Hence, we investigated two EF training conditions in children with ASD. Methods: In a randomized controlled trial, children with ASD (n = 121, 8-12 years, IQ > 80) were randomly assigned to an adaptive working memory (WM) training, an adaptive cognitive flexibility-training, or a non-adaptive control training (mock-training). Braingame Brian, a computerized EF-training with game-elements, was used. Outcome measures (pretraining, post-training, and 6-week-follow-up) were near-transfer to trained EFs, far-transfer to other EFs (sustained attention and inhibition), and parent's ratings of daily life EFs, social behavior, attention deficit hyperactivity disorder (ADHD)-behavior, and quality of life. Results: Attrition-rate was 26%. Children in all conditions who completed the training improved in WM, cognitive flexibility, attention, and on parent's ratings, but not in inhibition. There were no significant differential intervention effects, although children in the WM condition showed a trend toward improvement on near-transfer WM and ADHD-behavior, and children in the cognitive flexibility condition showed a trend toward improvement on near-transfer flexibility. Conclusion: Although children in the WM condition tended to improve more in WM and ADHD-behavior, the lack of differential improvement on most outcome measures, the absence of a clear effect of the adaptive training compared to the mock-training, and the high attrition rate suggest that the training in its present form is probably not suitable for children with ASD.