Purpose To examine the relationships between physical health problems, and borderline or clinical levels of mental health symptoms and children’s health-related quality of life (HRQoL). Methods Data were from the Longitudinal Study of Australian Children (2004–2018). Parents reported on their child’s HRQoL (PedsQL), physical health problems and mental health symptoms (Strengths and Difficulties Questionnaire, SDQ). A pooled cross-sectional analysis using linear regressions examined the relationships between physical health and clinical/borderline mental health symptoms, individually and when multi-morbid, and children’s HRQoL, and whether these relationships vary by a range of child, family and social factors. Results The sample comprised 47,567 observations of children aged 4–17 years. Borderline and clinical levels of mental health symptoms were associated with significantly lower HRQoL, equal to more than two-times (10.5 points) and more than three-times (16.8 points) the clinically meaningful difference, respectively. This was a larger difference than that associated with physical health problems (4.4 points). We found a significant interaction effect between physical health problems and clinical mental health symptoms which was associated with even poorer HRQoL after accounting for the individual relationships of both problems. Mental health problems were associated with poorer HRQoL for older versus younger children; and the interaction effect was significant for boys but not girls. Conclusion Findings highlight the importance of identifying and addressing mental health symptoms in children of all ages, even if these problems do not meet formal clinical criteria. Particular attention should be paid to the mental health and HRQoL of children with physical–mental multimorbidity, who are at risk of disproportionately poorer HRQoL.
