AIMS: Associations between growth differentiation factor-15 (GDF-15), cardiovascular outcomes, and exercise capacity among patients with a recent hospitalization for heart failure (HHF) and heart failure with reduced ejection fraction (HFrEF) are unknown. We utilized data from the 'Functional Impact of GLP-1 for Heart Failure Treatment' (FIGHT) study to address these knowledge gaps. METHODS AND RESULTS: FIGHT was a randomized clinical trial testing the effect of liraglutide (vs. placebo) among 300 participants with HFrEF and a recent HHF. Multivariable regression models evaluated associations between baseline GDF-15 and change in GDF-15 (per 1000 pg/mL increase from baseline to 30 days) with clinical outcomes (at 180 days) and declines in exercise capacity (6 min walk distance ≥ 45 m). At baseline (n = 249), median GDF-15 value was 3221 pg/mL (interquartile range 1938-5511 pg/mL). Participants in the highest tertile of baseline GDF-15 were more likely to be male and have more co-morbidities. After adjustment, an increase in GDF-15 over 30 days was associated with higher risk of death or HHF [hazard ratio 1.35, 95% confidence interval (CI) 1.11-1.64]. In addition, higher baseline GDF-15 (per 1000 pg/mL until 6000 pg/mL) and an increase in GDF-15 over 30 days were associated with declining 6 min walk distance (odds ratio 1.26, 95% CI 1.02-1.55 and odds ratio 1.37, 95% CI 1.12-1.69, respectively). GDF-15 levels remained stable among participants randomized to liraglutide. CONCLUSIONS: An increase in GDF-15 over 30 days among patients in HFrEF was independently associated with an increased risk of cardiovascular events and declining exercise capacity. These results support the value of longitudinal GDF-15 trajectory in informing risk of heart failure disease progression.