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.