BACKGROUND: People with cystic fibrosis face substantial physical, psychological, and social challenges as they move into adolescence and adulthood, which are likely to impact on their health-related quality of life. This study sought to examine the relative importance of physical and mental health variables associated with health-related quality of life in this group. METHODS: Adults and adolescents (N = 174; >= 14 y old) from across 11 adult or pediatric cystic fibrosis clinics in the Republic of Ireland, completed a background questionnaire that contained self-reported physical health variables, pulmonary function (ie, FEV1%) and body mass index. Questionnaire packs also contained the Hospital Anxiety and Depression Scale (HADS) and the Cystic Fibrosis Questionnaire-Revised, which has been specifically designed to assess health-related quality of life in patients with cystic fibrosis. RESULTS: HADS depression and/or anxiety scores were negatively associated with 11 of the 12 Cystic Fibrosis Questionnaire-Revised domain scores. FEV1% was positively associated with 8 domains when controlling for HADS anxiety but only 4 domains when controlling for HADS depression. HADS anxiety and depression scores demonstrated larger effect sizes and explained a greater proportion of the variance than pulmonary function in 8 of the 12 Cystic Fibrosis Questionnaire-Revised domain scores. CONCLUSIONS: Mental health variables, depression and anxiety, were strongly associated with health-related quality of life in subjects with cystic fibrosis and demonstrated greater effect sizes and explained a higher proportion of the variance overall than the physical health indicators, FEV1% and body mass index, which highlighted the importance of screening for, and treating, depression and anxiety symptoms.