<p>The unpredictability of recurrence of chronic subdural hematoma (CSDH) and in parallel its steady increase in incidence represents a global challenge. Recent evidence suggests the involvement of immune cells in its pathophysiology; however, little is known about the composition and relevance of individual immune compartments in the natural history of the entity. Herein we investigated the composition of the cellular component of the hematoma fluid in patients who underwent mini-craniotomy and compare it with autologous peripheral blood and that of healthy donor volunteers. <i>Results</i> The immunophenotypic analysis of the individual compartments showed that the CSDH infiltrate consists of immune cytotoxic cells predominantly T CD8 + lymphocytes. NK cells in CSDH patients were significantly lower in both central and peripheral compartments compared to healthy controls. Conversely, CD3 + CD56 + NK-T cells were higher in both central and peripheral compartments of CSDH patients compared to healthy controls. For the first time, an Erythro-Myeloid Progenitor population (CD45 + CD31+) was identified, with detailed analysis showing about 8.46% ± 9.12% for the CD45^highCD31^high subpopulation and 4.18% ± 1.47% for the CD45^lowCD31^low subpopulation. CD45^lowCD31^low cells were significantly absent in the peripheral blood of healthy controls compared to patients, and their value positively correlated with peripheral platelet count. Finally, inflammatory recruitment was evidenced by CX3CL1 levels, which were significantly higher in subdural fluid compared to patients’ peripheral blood and healthy controls. This pilot study provides, for the first time, the nature of the cellular composition of the lympho-leukocyte infiltrate in CSDH, with novel pathophysiological insights, and provides preliminary indications for the definition of a biomarker that correlates with clinical outcomes.</p>