<p>Abnormal electroencephalography (EEG) asymmetry has been implicated in several psychiatric disorders. However, the underlying mechanisms contributing to its dysregulation remain unclear. In this study, we investigated whether the proportions of leucocyte subpopulations are associated with relative EEG asymmetry across the frontal, central, parietal, and occipital brain regions. We recorded resting-state EEG from 24 healthy participants and calculated relative EEG asymmetry indices following standard preprocessing procedures. Peripheral blood samples were collected from each participant, and DNA was extracted for genome-wide DNA methylation analysis. DNA methylation data were used to estimate the relative proportions of leucocyte subpopulations. Pearson’s correlation was used to identify associations between EEG asymmetry and leucocyte compositions. To account for multiple comparisons, p-values were adjusted using the false discovery rate (FDR) method. We identified several associations between lymphocyte subpopulations and EEG asymmetry. CD8 + T cells were associated with occipital beta asymmetry (<i>p</i> = 0.009), while CD4 + T cells were linked to frontal alpha, frontal delta, occipital alpha, and occipital delta asymmetry (p-values ranging from 0.017 to 0.023). Natural killer cells showed an association with frontal beta asymmetry (<i>p</i> = 0.052), and B cells with occipital beta asymmetry (<i>p</i> &lt;0.001). Granulocytes were associated with occipital beta and occipital delta asymmetry (p-values ranging from 0.01 to 0.032). However, none of these associations remained significant after FDR correction. The identified associations between the leucocyte subpopulation proportions and EEG asymmetry may be partially mediated by cytokine signaling within the central nervous system. Gaining insight into how EEG asymmetry is regulated in healthy individuals may aid future investigations focusing on its dysregulation in psychiatric populations.</p>