OBJECTIVES: Iron deficiency (ID) is frequent in heart failure (HF). Among patients with HF and ID, those with impaired iron transport (IIT) (transferrin saturation (TSAT) <20%) have the worst ID phenotype. In HF, exercise limitation is mainly related to abnormality in oxygen delivery (VO2) and utilisation and/or to ventilation inefficiency. We evaluated whether it is possible to identify the leading cause of exercise limitation in patients with HF and IIT. DESIGN: Observational study. SETTING: Retrospective study. PARTICIPANTS: We analysed 1043 consecutive hospitalised patients with HF (66±14 years, 49.8% females) who underwent cardiopulmonary exercise test (CPET). OUTCOME MEASURES: Associations between CPET parameters and TSAT were explored using general linear models adjusted for potential confounders (haemoglobin, left ventricle ejection fraction, age, gender, C reactive protein, serum creatinine). RESULTS: We observed that: (a) 413 patients with HF and IIT had worse functional capacity compared with non-IIT cases: peak VO2 (15±3 vs 16±6 mL/min/kg, p<0.0001) and ventilation/carbon dioxide relationship (VE/VCO2) slope (38±9 vs 33±8, p<0.0001); (b) VE/VCO2 values remained significantly different between IIT and non-IIT cases after adjusting for confounding variables including peak VO2; differently, peak VO2 after adjusting also for VE/VCO2 slope, resulted not different in IIT compared with non-IIT HF cases; (c) patients with both low peak VO2 (<14 mL/min/kg) and high VE/VCO2 (≥34) had a higher B-type natriuretic peptide (BNP), lower TSAT and higher MECKI (Metabolic Exercise combined with Cardiac and Kidney Index) score compared with patients with high peak VO2 and low VE/VCO2 (BNP 951±1041 vs 413±623 pg/mL (p<0.0001); TSAT 19.5%±10.5% vs 26.9%±10.2% (p<0.0001); MECKI score 14.8%±1.4% vs 1.2%±2.3% (p<0.0001). CONCLUSIONS: High VE/VCO2 slope is directly associated with IIT, independent of peak VO2, suggesting a pivotal role for ventilation inefficiency in exercise impairment in patients with IIT and HF. In patients with HF, the worst exercise performance is associated with low TSAT, high BNP and the highest MECKI score.