The association between glycaemic control of type 1 diabetes and mental health issues within the family is well recognised, but the degree of difficulty that is associated with modest sub-optimal control is not. 66 families agreed to take part in the study, and they were divided into two groups according to the quality of metabolic control (good or sub-optimal). Of the 37 boys, 19 had suboptimal metabolic control, and of the 27 girls, 15 did not have a good control. Mother and child participants provided demographic information, and mothers completed the 28-item General Health Questionnaire (GHQ), the revised version of the Swanson, Nolan and Pelham Questionnaire (SNAP IV), elements of the Ontario Child Health Scale, and the Family Adaptation and Cohesion Evaluation Scale (FACES IV). The results from these scales were compared to the quality of the children’s glycaemic control. Sub-optimal control was associated with the increased emotional symptoms and behavioural difficulties in the young person (p <0.0001), and increased mental health difficulties in their mothers (p <0.001). These elements had an adverse impact on the level of satisfaction with family life. Having sub-optimal glycaemic control is associated with an increased risk of having mental health symptoms, and a possible mechanism for this association is explored. Even modest deterioration in a family’s mental health can have an adverse effect upon glycaemic control, and should be assessed as a routine in review clinics.