Research shows that depression and anxiety disorders are the most common psychiatric concern in autism spectrum disorders (ASD). Mindfulness-based therapy (MBT) has been found effective in reducing anxiety and depression symptoms, however research in autism is limited. Therefore, we examined the effects of a modified MBT protocol (MBT-AS) in high-functioning adults with ASD. 42 participants were randomized into a 9-week MBT-AS training or a wait-list control group. Results showed a significant reduction in depression, anxiety and rumination in the intervention group, as opposed to the control group. Furthermore, positive affect increased in the intervention group, but not in the control group. Concluding, the present study is the first controlled trial to demonstrate that adults with ASD can benefit from MBT-AS. (Contains 2 tables and 1 figure.)