Abstract Aims Iron deficiency is common in heart failure with reduced ejection fraction (HFrEF). In patients with cardiac resynchronization therapy (CRT), it is associated with a diminished reverse remodelling response and poor functional improvement. The latter is partially related to a loss in contractile force at higher heart rates (negative force–frequency relationship). Methods and results The effect of intravenous ferric carboxymaltose on reverse remodelling following cardiac resynchronization therapy (IRON‐CRT) trial is a multicentre, prospective, randomized, double‐blind controlled trial in HFrEF patients who experienced incomplete reverse remodelling (defined as a left ventricular ejection fraction below