Background: Mechanical thrombectomy is standard of care for acute ischemic stroke secondary to LVO however MT failure (MTF) occurs in approximately 15% of cases. Objective: Investigate patient and procedural characteristics that predict MTF. Methods: Retrospective review of prospectively collected, Stroke Thrombectomy and Aneurysm Registry. Patients who underwent MT for LVO were included and categorized by MT Success (MTS) or MTF, defined as mTICI≥2b or <mTICI2b, respectively. Demographics, pretreatment, and treatment information were included in UVA and MVA for prediction of MTF. Results: A total of 8452 patients were analyzed, and 1301(15.4%) experienced MTF.MTF patients were older(73 vs.71, p=0.008)had higher poor premorbid mRS(10.8% vs 8.4%, p=0.17).Onset to puncture was greater in MTF group(442 vs.411 min, p=0.006).There were more ICA occlusions(15.6% vs. 13.5%)and basilar occlusions(7.8% vs.6.2%)in the MTF group and more M1 occlusions(42.2%vs.37.5%)in the MTS group (p<0.001).More patients underwent aspiration as the final technique in the MTS group(35.3% vs 32.9%).Number of passes(3 vs 2)and procedure time (77.3 vs 46.1 min) were higher in the MTF group (p<0.001). More patients in the MTF group required IA thrombolytic(14.7% vs. 8.3%, p<0.001).More patients in the MTS group had rescue stenting(7.9% vs 4.8%).There were more complications (14.7% vs 6.2%) and more symptomatic ICH(9.9% vs 5.7%, p<0.001)in the MTF group. Favorable outcome at 90 days was greater in the MTS group(42.6% vs 18.3%, P<0.001).On UVA, age, poor pretreatment mRS, posterior circulation occlusion, final technique SR, increased number of passes, increased procedure time were associated with increased odds of MTF, while M1-M2 occlusions and rescue intracranial stenting with decreased odds of MTF. These correlations remained significant on MVA for final technique SR, rescue intracranial stenting, number of passes, and procedure time. Conclusion: MTF is associated with more complications and worse outcome. Final use of aspiration and rescue intracranial stenting may increase chances of recanalization.