This study examined the adjustment of siblings of children with mental health problems. The participants had brothers or sisters receiving treatment at a Child and Adolescent Mental Health Service within the Hunter New England Health Service, New South Wales, Australia. Seventy-five siblings completed questionnaires on their self-concept, quality of life and family functioning. Their carers completed questionnaires on the sibling's behaviours, family demographics and family functioning. The diagnosis and severity of disability of index children were attained from the treating clinician. The study revealed the siblings had significantly higher rates of psychopathology, poorer quality of life and lived in more dysfunctional families than normally developing children. Regression modelling predicted variables related to the three main behaviour scales of the Child Behaviour Checklist (CBCL). Family functioning, family communication and the extreme ends of family cohesion and balance, were the most significant contributors to regression model. The sibling self-concept domains of global self-worth, athletic competence and behavioural conduct, contributed to the model. Additionally, a smaller family size was associated with more internalising behaviour disorders in the siblings. The diagnosis and severity of mental health problem of the index child and other family demographic factors did not impact on the sibling's behavioural functioning. This study highlights that siblings of children with mental health difficulties are a group vulnerable to adjustment difficulties regardless of the index child's diagnosis or the severity of impairment. Overall, family functioning had a greater impact on the siblings than other factors.