BACKGROUND: Bleeding and thrombosis are major complications of veno-venous extracorporeal membrane (VV-ECMO). OBJECTIVES: To assess thrombosis, major bleeding (MB) and 180-day in patients supported by VV-ECMO between first (1st March-31st May 2020) and second (1st June 2020-30th June 2021) waves of the COVID-19 pandemic. PATIENTS/METHODS: Observational study of 309 consecutive patients (≥18years) with severe COVID-19 supported by VV-ECMO in four nationally commissioned ECMO centres, UK. RESULTS: Median age was 48 (19-75)years and 70.6% were male. Probabilities of survival, thrombosis, and MB at 180 days in the overall cohort were 62.5% (193/309), 39.8%(123/309) and 30%(93/309). In multivariate analysis, age >55 years (HR 2.29 [1.33-3.93],p=0.003) and elevated creatinine (HR 1.91 [1.19-3.08],p=0.008) were associated with increased mortality. Corrected for duration of VV-ECMO support, arterial thrombosis alone (HR 3.0 [95% CI1.5-5.9], P= 0.002) or circuit thrombosis alone (HR 3.9 [95% 2.4-6.3], P<0.001), but not venous thrombosis, increased mortality. MB during ECMO had 3-fold risk (95% CI 2.6-5.8, P<0.001) of mortality. The first wave cohort had more males (76.7% vs 64%, p=0.014), higher 180-day survival (71.1% vs 53.3% p=0.003), more venous thrombosis alone (46.4% vs 29.2%, p=0.02) and lower circuit thrombosis (9.2% vs 28.1%, p<0.001). The second wave cohort received more steroids (121/150 [80.6%] vs 86/159 [54.1%], p<0.0001) and Tocilizumab (20/150 [13.3%] vs 4/159 [2.5%] p=0.005). CONCLUSIONS: MB and thrombosis are frequent complications in patients on VV-ECMO and significantly increase mortality. Arterial thrombosis alone or circuit thrombosis alone increased mortality whilst venous thrombosis alone had no effect. MB during ECMO support increased mortality 3.9-fold.