PurposeThis paper aims to investigate parents' beliefs about the causes of their child's Type 1 diabetes to understand if this affects the way diagnosis is processed and if this impacts on sibling parenting.Design/methodology/approachOnline, semi-structured qualitative interviews with nine parents of children with Type 1 diabetes who have at least one non-diabetic child. The results were analysed using interpretative phenomenological analysis (IPA).FindingsTwo interlinked themes were identified: “What ifs”: parents postulated underlying genetic reasons for their child's diabetes and had working theories about the triggers of diabetes that included stress, infection, vaccination or a virus. Developing a personal aetiology of their child's condition allowed some a feeling of control, while others focused on practical ways to manage diabetes. “Having something to blame”: narratives dwelt on the relationship between beliefs about causes and self-blame. Some believed that acting on an identified trigger reduced personal guilt.Research limitations/implicationsAlthough internet access is widespread in the UK, a limitation of this research is that it excluded those without internet access.Practical implicationsThe findings of this research may provide greater depth and a more holistic perspective to the health promoter to better support parents of Type 1 diabetics.Social implicationsThe analysis of illness narratives that this research provides may offer a greater understanding of the social context in which health and illness develop. This research found some examples of parental confidence about the causes and triggers of their child's diabetes being positively associated with a sense of control. This might indicate the value of a more comprehensive larger-scale study to establish whether parents who are supported to develop a personalised conception of the aetiology of their child's diabetes develop a greater sense of coherence and well-being regarding their child's condition.Originality/valueThere is very limited literature focusing on the beliefs of sufferers and their families about Type 1 diabetes causality. Of that which does exist, some research is heterogenous in its sampling of Types 1 and 2 diabetes sufferers. This study offers a rare, focused insight into the beliefs of parents about the background causes and more proximal triggers of their child's Type 1 diabetes.