Introduction. Heart failure (HF) is a significant clinical and socioeconomic problem both in Poland and around the world. However, objective data on the level of adherence in the era of improved medical therapy is lacking. Therefore, the aim of the study was to investigate the level of medication adherence and its association with demographical and clinical variables in patients with HF. Material and methods. We have conducted a prospective cohort study of 25 patients with diagnosed HF. Medication adherence was measured for 30 consecutive days using the Medication Event Monitoring System (MEMS) — an electronic cap attached to the medication container, allowing to record the exact moment of taking the measured medicine. Based on the acquired data, patients were classified as adherent or non-adherent using an evidence-based cut-point. In addition to adherence measurement, patients’ demographic and clinical information was collected. Results. Twenty-two patients provided full results from the MEMS devices. The median age of the patients was 70 years (interquartile range =14), and the mean left ventricular ejection fraction was 33% ± 12. The mean percentage of correct doses was 89% ± 17. Twenty-seven percent of patients (n = 6) were classified as non-adherent. Patients classified as non-adherent were significantly younger (54 vs. 71 years; p = 0.015), had a lower left ventricular ejection fraction (24 vs. 36%; p = 0.04), and were more frequently enrolled after HF hospitalization (83 vs. 19%; p = 0.011). Conclusions. In the short-term observation, a significant proportion of patients with HF were found to be non-adherent. In our study, we identified a population with an increased risk of non-adherence. Those patients require the implementation of more intensive and targeted healthcare system-based interventions in order to improve their prognosis.