Hypothesis: We aim to identify a profile of intracranial thrombus resistant to recanalization by standard mechanical thrombectomy (MT) in acute stroke treatment. Methods: First extracted clot of each MT were analyzed by Flow Cytometry obtaining composition of main leukocyte populations: granulocytes, monocytes and lymphocytes. Demographics, reperfusion treatment and grade of recanalization were registered. MT Failure ( MTF) was defined as final Thrombolysis in Cerebral Infarction score IIa or lower and/ or need of permanent intracranial stenting as a rescue therapy after standard MT. In other cohort of cases, unconfined compression tests were performed to explore stiffness of retrieved clots . We looked for correlation between mechanical characterization tests and clot composition. Results: Among 225 patients, there were 13 % of MTF that were significantly associated to atherosclerosis etiology ( 33.3% vs. 15.9% ; p 0.021) , more passes ( 3 vs. 2; p Conclusions: There is a positive correlation between granulocyte proportion and thrombi stiffness that may explain endovascular resistance to recanalization. Influence of granulocytes within thrombus may be a target for future reperfusion treatments.