Research from the general population indicates an important role for self-esteem in mental health, but limited research in this area exists in the cystic fibrosis (CF) literature. This study aimed to explore the predictive value of self-esteem and health-related quality of life (HRQoL) in mental health symptoms in adults with CF. Seventy-four participants, recruited online, completed the Clinical Outcomes in Routine Evaluation-Outcome Measure 34 (CORE-OM), Rosenberg Self-esteem Scale and Cystic Fibrosis Questionnaire-Revised (CFQ-R). Comparably high levels of self-esteem were found, but HRQoL was lower than previous research. Thirty percent of participants scored within the clinical range for mental health difficulty. Hierarchical regression, controlling for gender, explored the value of four CFQ-R subscales (physical, social, emotional and role functioning) and self-esteem in predicting CORE-OM total score. Gender accounted for 8.2% of the variance in mental health scores while the five independent variables accounted for a further 73.0% of variance. Of the five variables, CFQ-R emotional functioning and self-esteem were significant predictors of mental health symptoms. Results are discussed in relation to clinical implications and potential uses for internet technologies to promote socialisation.