ObjectiveTo identify predictors of change in lung function and body weight during healthcare transition in cystic fibrosis (CF). MethodsWe conducted a retrospective cohort study using data from the CF Foundation Patient Registry (CFFPR) and the web-based transition program CF RISE (Responsibility. Independence. Self-care. Education) for patients ages 16 to 25 who transitioned to adult care from 2013 through 2019. We modeled change in FEV1 % predicted and weight using linear regression fit with generalized estimating equations. Predictors included gap in care (time between last pediatric and first adult outpatient visit), transition program engagement, and sociodemographic and medical factors. ResultsAmong 12,420 adolescents and young adults (AYAs), 3,876 transitioned to adult care with a median gap in care of 7.6 months. Patients from CF centers with higher rates of CF RISE engagement had improved lung function and weight at their first adult outpatient visit. Coverage on a parent’s insurance plan and absence of CF complications predicted increased lung function. History of a non-lung transplant and sinus disease predicted increased weight. Comorbid diabetes mellitus and gaps in care >3 months predicted decreased lung function with longer gaps in care associated with greater decrease. A gap in care of 6-9 months predicted decreased weight. Control variables including baseline FEV1 and weight, and exacerbation status were also statistically significant. ConclusionsFindings suggest two promising targets to improve transition of AYAs with CF: increasing AYA engagement in CF RISE and reducing gaps in care during the transition period.