Background: The efficacy of a thrombectomy procedure for acute ischaemic stroke (AIS) is largely dependent on mechanical behaviour of thrombi and interactions with the thrombectomy device. Studies have examined clot analogues and evidence suggests that thrombus mechanical properties largely depend on composition, yet evidence based on thrombi retrieved from AIS patients is still lacking. Therefore, this study aims to characterize the mechanical properties of thrombi retrieved from AIS patients and to determine the relation to thrombus composition. Additionally, results were compared to literature in order to assess clot analogue representativeness.Methods: Directly following a thrombectomy procedure, unconfined compression tests were performed on thrombi retrieved from acute ischaemic stroke patients. For all tested samples, the material properties were characterized and related to the histologically determined composition. Identified histological components were 1) Fibrin & platelets, 2) Red blood cell and 3) Leukocytes. A subgroup analysis was performed to compare values with literature, where samples were stratified into four groups based on fibrin & platelet content (F&P -low, -moderate low, -moderate high and -high). Results: A total of 18 patients yielded 39 samples which were successfully tested and histologically analysed. Sample stiffness was found to be positively correlated to fibrin & platelet content (Rs=0.69, p<0.001). A good histological distribution was present within the data, as the fibrin & platelet content ranged from 7% to 99%. Subgroup analysis showed little difference in mechanical behaviour between the F&P moderate-low and F&P moderate-high subgroups, with the F&P low and F&P high groups respectively exhibiting a decreased and increased stiffness. Comparing to current literature, the results demonstrated that analogues most accurately resemble thrombi with a low fibrin & platelet content. Furthermore, all samples displayed viscoelastic and non-linear stress-strain behaviour.Conclusion: It was found that composition is a strong influencing factor of thrombus mechanical properties. Both at high and low fibrin & platelet contents, the relation between composition and stiffness was strongest, while it was least pronounced at moderate fibrin & platelet contents (approximately 25%-75%).