Ptaszynska–Kopczynska K, et al. – The target of this research was to investigate the interleukin–6 (IL–6) system, with the inclusion of soluble IL–6 receptor (sIL–6R) and soluble glycoprotein 130 (sgp130), in heart failure with reduced ejection fraction (HF–REF) patients, with a focus on cardiac resynchronization therapy (CRT) effects. The patients reported higher IL–6 and lower sIL–6R levels. IL–6 concentration reflected their clinical status. A link was observed between CRT–related improvement of patients’ functional status with a smaller change of sIL–6R concentration in time.