Although research suggests that posttraumatic stress disorder symptoms in relation to physical health diagnoses may be related to poor adherence to treatment regimens, so far, whether parental posttraumatic stress disorder symptoms have an impact on their child’s adherence to insulin-dependent diabetes mellitus treatment has not been investigated. Using self-report questionnaires, the present study found that children of mothers who have posttraumatic stress disorder symptoms in relation to their child’s diagnosis of type I diabetes showed poorer adherence to treatment than the children of mothers without posttraumatic stress disorder. However, this was only the case for younger children (aged 0–8 years) where mothers played a more active role in their child’s treatment.