Extracorporeal lung support includes the risk of hemolysis due to suction pressures. Manufacturers measure the negative suction pressure across drainage cannulas for their products in vitro using water. Clinical experience suggests that hemolysis occurs in vivo already at much lower flow rates. The aim of this study was to analyze the in vivo suction pressure for veno-venous extracorporeal membrane oxygenation (VV-ECMO) cannulas. Prospective, observational study at a tertiary-care intensive care unit: 15 patients on VV-ECMO for severe ARDS were prospectively included. In vitro, the 25 Fr drainage cannula pressure drops below a critical level of around –100 mm Hg at a flow rate of 7.9 L/min, the 23 Fr drainage cannula at 6.6 L/min. In the clinical setting, critical suction pressures were reached at much lower flow rates (5.5 and 4.7 L/min; p < 0.0001, nonlinear regression). The in vitro data largely overestimate the safely achievable flow rates in daily clinical practice by 2.4 L/min (or 44%, 25 Fr) and 1.9 L/min (or 41%, 23 Fr). In vivo measurement of suction pressure of venous drainage cannulas differed significantly from in vitro derived measurements as the latter largely underestimate the resulting suction pressure.