Objectives: The aim of this study was to determine the dynamics of CD56dimCD16++DNAM-1+ NK cells and CD16++DNAM-1+ monocytes in patients with ST-elevation myocardial infarction (STEMI) in the early period after of treatment with primary percutaneous coronary intervention (PCI). An additional goal was to investigate the relationship between the number of subpopulations of these cells and the left ventricular ejection fraction (LVEF). Patients and methods: A total of 44 patients with a diagnosis of STEMI who underwent primary PCI at the University Hospital of Split, in the period from June 2018 to April 2021, were included in the study. The patients' blood was sampled upon admission and 3 hours after and 24 hours after the PCI procedure.. A flow cytometer (BD Accuri C6, BD Biosciences, Belgium) was used to analyze cell subpopulations. Results: There is a decrease in the number of CD56dimCD16++DNAM-1+ NK cells (P=0.002) and CD16++DNAM-1+ monocytes (P=0.003) in patients' peripheral blood 3 hours after PCI, with a return to initial values 24 hours after the procedure. A negative correlation was observed between the number of CD56dimCD16++DNAM-1+ NK cells at admission and LVEF (r=-0.425, P=0.001), while no statistically significant correlation was demonstrated at other time points. Also, no correlation between LVEF and monocyte subpopulation was shown at any time point. Conclusions: The research showed the dynamics of the decrease in the number of NK cell subpopulation and the number of monocyte subpopulation 3 hours after PCI and their return to initial values after 24 hours. We believe that the reason for this is their migration to the infarcted area of the myocardium. Furthermore, a negative correlation between the number of CD56dimCD16++DNAM-1+ NK cells at admission and LVEF was observed, which may represent the basis for future research aimed at developing a therapeutic approach through the prism of DNAM-1 function.