Background: Previous studies have reported correlations between various aspects of the behaviour and symptomatology of children with Autism Spectrum Disorder (ASD) and their parents' self-reports of stress via standardised scales. Aims: To extend that literature, a physiological index of parental chronic stress was used instead of their self-reports-dysregulation of the Diurnal Rhythm (DR) of the Hypothalamic-Pituitary-Adrenal (HPA) axis. Methods: A sample of 149 parents of a child with ASD provided salivary cortisol at the predicted time of daily maximum cortisol concentration and at a time of daily lower concentration. Adherence to the predicted DR was assessed via a dichotomous (present/not-present) as well as a continuous measure, and MANOVA and linear regression were used to detect significant associations between ASD-related variables in their children and parents' DR. Results: Identified only a single significant correlate of DR dysregulation in both statistical procedures-Self-Injurious Behaviour (SIB) exhibited by their child and observed by the parents. Conclusions and Implications: These findings extend previous data using self-report indices of parental stress and should be included in parent-support settings to alert parents to the long-term health effects of the stress they experience in regard to their child's SIB.