The objective was to evaluate the predictive value of the ROX index and describe the evolution of a population of patients admitted to intensive care for COVID-19 pneumonia who required high-flow oxygen therapy.Retrospective cohort study in patients older than 18 years with a positive nasopharyngeal swab for SARS-COV-2 who were admitted to intensive care unit with acute respiratory failure and required high-flow oxygen therapy for > 2 hours.Of a total of 97 patients, 42 (43.3%) responded satisfactorily to treatment with high-flow nasal cannula (HFNC) and 55 (56.7%) failed treatment, requiring orotracheal intubation and invasive ventilatory support. Of the 55 patients who failed, 11 (20%) survived and 44 (80%) died during intensive care admission (p < 0.001). No patient who responded satisfactorily to HFNC treatment died during hospitalization. The ROC analysis identified the 12-hour ROX index as the best predictor of failure with an area under the curve of 0.75 (0.64-0.85) and a cut-off point of 6.23 as the best predictor of intubation [Sensitivity 0.85 (95% CI 0.70-0.94), Specificity 0.55 (95% CI 0.39-0.70)].In patients with acute respiratory failure secondary to COVID-19 pneumonia treated with highflow oxygen therapy, the ROX index was a good predictor of success.