This study examined the relationship of medication adherence to frequency of pulmonary exacerbation and rate of decline in FEV 1 % predicted (FEV 1 ).95 CF patients aged 6years or older and prescribed a pulmonary medication, were enrolled in a longitudinal retrospective review of medication adherence and health outcomes (the occurrence and frequency of intravenous (IV) antibiotic treatments and FEV 1 ) over 12-months. Pharmacy refill records were used to calculate a medication possession ratio (MPR).Composite MPR predicted the occurrence of at least one pulmonary exacerbation requiring a course of IV antibiotics (IRR=2.34, p=0.05), but not the frequency of exacerbations, after controlling for gender, baseline FEV 1, and regimen complexity. Composite MPR predicted baseline FEV 1 (estimate=29.81, p=.007), but not decline in FEV 1 .These results demonstrate a significant relation between medication adherence and IV antibiotics in CF patients, highlighting the importance of addressing adherence during clinic visits to improve health outcomes.