Our goal was to determine the predictive role of the combined assessment of the vasoactive-inotropic score (VIS) and lactate levels for the prognosis of patients with postcardiotomy cardiogenic shock (PCS) requiring venoarterial extracorporeal membrane oxygenation (VA-ECMO). The data of adult patients with PCS requiring VA-ECMO between January 2015 and December 2018 at a tertiary hospital were analysed retrospectively. The incidence of in-hospital mortality and other clinical outcomes was analysed. The associations of the VIS and the lactate concentration and in-hospital mortality were assessed using logistic regression analysis. A total of 222 patients were included and divided into 4 groups according to the cut-off points of the VIS (24.3) and the lactate level (6.85 mmol/L). The in-hospital mortality rates were 37.7%, 50.7%, 54.8% and 76.5% for the 4 groups (P < 0.001), and the rates of successful weaning off VA-ECMO were 73.9%, 69%, 61.3% and 39.2%, respectively (P = 0.001). Groups 1 and 2 exhibited significant differences compared to group 4 in both in-hospital mortality and weaning rates (P < 0.05). There was a statistically significant difference in the incidence of multiple organ dysfunction between group 1 and group 4 (P < 0.05). Groups 1, 2 and 3 demonstrated significantly improved cumulative 30-day survival compared with group 4 (log-rank test, P < 0.05). Logistic regression analysis revealed that age, a VIS > 24.3 and lactate levels > 6.85 mmol/L were independently predictive of in-hospital mortality. Among patients with PCS requiring VA-ECMO, the initiation before reaching a VIS > 24.3 and lactate levels > 6.85 mmol/L was associated with improved in-hospital and 30-day outcomes, suggesting that the combined assessment of the VIS and lactate levels may be instructive for determining the initiation of VA-ECMO.