Background: Psychological mechanisms may help to explain the variance observed in parental psychological adjustment in parents of children with intellectual disability (ID). In this study, parental locus of control and its role in relation to maternal psychological well-being was explored. Method: Questionnaires were sent to 91 mothers of children with ID at two time points, 18 months apart. Results: Parental locus of control was associated with both maternal positive perceptions and with maternal distress. Regression analyses showed that dimensions of parental locus of control were significant predictors of negative maternal adjustment. Maternal positive perceptions were predicted by perceived control of the child and belief in fate or chance. Overall parental internal-external locus of control entered into a bidirectional relationship with stress over 18 months. Conclusions: Parental locus of control is a construct that may explain some of the variance in maternal well-being and thus is a construct that merits further research. (Contains 5 tables and 1 note.)