Background: The benefit of antibiotic treatment of acute drops in FEV 1 percent predicted (FEV 1pp) has been clearly established, but data from the early 2000’s showed inconsistent treatment. Further, there is no empirical evidence for what magnitude of drop is clinically significant. Methods: We used data from the CF Foundation Patient Registry (CFFPR) from 2016-2019 to determine the association between treatment (any IV antibiotics, only oral or newly prescribed inhaled antibiotics, or no antibiotic therapy) following a decline of ≥5% from baseline FEV 1pp and return to 100% baseline FEV 1pp days using multivariable logistic regression including an interaction between the magnitude of decline and treatment category. Results: Overall, 16,495 PWCF had a decline : 16.5% were treated with IV antibiotics, 25.0% non-IV antibiotics, and 58.5% received no antibiotics. Antibiotic treatment was more likely for those with lower lung function, history of a positive PA culture, older age and larger FEV 1 decline (p<0.001). Treatment with IV antibiotics or oral/inhaled antibiotics was associated with a higher odds of recovery to baseline compared to no treatment across all levels of decline, including declines of 5-10%. Conclusions: A large proportion of acute drops in FEV1 pp continue to be untreated, especially in younger patients and those with higher baseline lung function. Acute drops as small as 5% predicted are less likely to be recovered if antibiotic treatment is not prescribed. These findings suggest the need for more aggressive antimicrobial treatment of acute drops in FEV 1, including those of a magnitude previously believed to be associated with self-recovery.