Purpose <p>Many Autistic individuals present with comorbid conditions, including internalising and externalising behaviours, sleep issues, intellectual disabilities, and gastrointestinal dysfunction. We investigated the impact of these child comorbidities on parenting stress in an effort to elucidate the underlying mechanism and how they interact with autistic core symptoms. In total, three theoretical models were tested, being the Amplification, Additive, and Mediation Hypotheses.</p> Methods <p>Participants were 453 parents of an Autistic child reporting on their child’s core symptoms, comorbid conditions, and their parenting stress.</p> Results <p>Correlation analyses reveal moderate associations between the comorbid conditions and parenting stress, but uncovered a weak link between core symptoms and parenting stress. Regression analyses revealed that, when key variables were allowed to adjust for one another, comorbid conditions were found to be independent predictors of parenting stress. A subsequent path analysis indicated that internalising and externalising behaviours partially mediated the relationship between core symptoms and parenting stress. There was no evidence to support the Amplification Hypotheses, and limited evidence to support the Additive and Mediation Hypotheses.</p> Conclusion <p>The findings reinforce the argument that Autistic children require multidisciplinary services and interventions that stretch beyond their primary diagnosis. Further suggestions for future research into child comorbid factors and parenting stress are discussed.</p>