Few attempts have been made to evaluate the effectiveness of parent-training (PT) based on mindfulness approaches for parents of children with autism. We present findings of a study on an Acceptance and Commitment Therapy (ACT)-oriented PT with a specific focus on the improvement of parents’ psychological functioning. Stress and two psychological processes related to emotional suffering in accordance to the framework of ACT (namely, cognitive fusion and experiential avoidance) were assessed in parents before and after treatment. The effects of treatments contemporarily received by children were controlled by comparing an experimental group that received the ACT-PT along with an early intensive behavioral intervention (EIBI) for the child, with a control group which only received the EIBI. Multivariate analysis of variance showed a paradoxical reduction of mindfulness awareness after treatment (p < .01) and a trend towards statistical significance of the change in stress (p = .06) in the group receiving the ACT-PT. No effect of treatments (ACT-PT + EIBI vs. EIBI only) was found on cognitive fusion. We discussed the results while considering the possible changes in psychological awareness promoted by ACT. Parents of the experimental group could have become more aware of their inner states, which may explain the unexpected negative change in mindfulness awareness and the limited decrease of stress after treatment. Moreover, questionnaires assessing the ACT-based processes were likely to be too complicated for the parents to understand; therefore, comparisons between pre- and post-treatment measures may not be entirely reliable. Methodological challenges and indications for future research are highlighted.