BACKGROUND: Cardiorespiratory fitness (CRF) is closely linked to health status and clinical outcomes in heart failure (HF) patients. We aimed to test whether biomarkers can reflect CRF and its change over time. METHODS: This post hoc analysis used data from ambulatory cohorts of heart failure with reduced ejection fraction (HFrEF) (IRONOUT) and heart failure with preserved ejection fraction (HFpEF) (RELAX). Cardiopulmonary exercise testing, 6-minute walk distance (6MWD), and serum biomarkers were measured at baseline and 16- or 24-week follow-up (for IRONOUT and RELAX respectively). Biomarkers included N-terminal pro-B-type natriuretic peptide (NT-proBNP), soluble ST2, growth differentiation factor-15, and Galectin-3. RESULTS: Analysis included 225 patients with HFrEF and 216 with HFpEF. Baseline peak VO2, VE/VCO2 slope, and 6MWD showed a mild correlation with the doubling of all 4 tested biomarkers in HFrEF and HFpEF. Following multivariable adjustment (including all biomarkers), the only significant association between change in biomarker and functional parameter in HFrEF was change in NT-proBNP and change in VE/VCO2 slope (3.596% increase per doubling, 95% CI 0.779-6.492, P = .012). In HFpEF, a decrease in peak VO2 was associated with an increase in NT-proBNP (-0.726 mL/min/kg per doubling, 95% CI -1.100 to -0.353, P < .001), and a decrease in 6MWD was associated with an increase in growth differentiation factor-15 (-31.606 m per doubling, 95% CI -61.404 to -1.809, P = .038). CONCLUSIONS: In these ambulatory trial cohorts, NT-proBNP was associated with baseline and change in CRF in HFrEF and HFpEF. In contrast, novel biomarkers do not appear suitable as a reliable surrogate for serial assessment of exercise capacity in HF patients given lack of consistent independent association with CRF beyond traditional risk factors and NT-proBNP.