BackgroundSeveral nongenetic factors, such as socioeconomic status, environmental exposures, and adherence have been described to have an impact on outcomes in cystic fibrosis (CF).ObjectiveTo determine the effect of social complexity on the % predicted forced expiratory volume (ppFEV1) before and after transfer to adult care among adolescents with CF.MethodsRetrospective, single center, cohort study included all patients with CF who were transitioned into adult care between 2005 and 2015 at Indiana University. Social complexity (Bob's level of social support, [BLSS]) was collected at transfer. Linear mixed regression models assessed the relationship between ppFEV1 decline over time and BLSS with other covariates.ResultsThe median age of the patients (N = 133) at the time of transfer was 20 years (interquartile range: 19‐23). Overall, there was a decline in lung function over time in our population (ppFEV1 at 24 months pretransfer 77 ± 20%, ppFEV1 at 24 months, posttransfer 66 ± 24%; P < .001). The relationship between BLSS and ppFEV1 became more negative over time, even after adjusting for other covariates.ConclusionSocial complexity is strongly associated with lung function decline after transfer to adult care.