Background: There is a body of evidence that indicates that the cognitions of parents of children with intellectual disabilities (ID) play an important role in influencing parental stress. However, there is a paucity of evidence about the experience of parents of adult children with ID. This study sought to apply a model of parenting stress to mothers of adults with ID. Of particular interest were the parental cognitions of parenting self-esteem and parental locus of control. Method: Face-to face interviews were administered with 44 mothers of adults with ID. They completed the Vineland Adaptive and Maladaptive Behaviour Scale, the Family Support Scale, the Parenting Sense of Competence Scale, a shortened version of the Parental Locus of Control Scale and the Parenting Stress Index. Results: Correlations were observed between parenting stress and the other study variables. Regression analysis revealed that parental cognitive variables predicted 61% of the variance in parenting stress. Parenting satisfaction, a subscale of the measure of parenting sense of competence, mediated the relationships between adaptive behaviour and parenting stress and between family support and parenting stress. Conclusions: These results indicate the importance of cognitive variables in the stress of mothers of adults with ID. Potential avenues of future research might focus on the experience of fathers and the impact of positive perceptions as a cognitive factor.