Abstract Background Yellow Fever (YF) is still a major threat in developing countries and a cause of outbreaks in Africa and Latin America, despite a highly efficacious vaccine. In 2018, the Brazilian state of São Paulo witnessed a new YF outbreak in areas where the virus has not been detected before. In our study, we included all patients who were admitted to Intensive Care Units of Hospital das Clínicas, University of São Paulo Medical School during the 2018 YF outbreak. The aim is to describe the clinical and laboratorial characteristics of severe cases of YF, evaluate viral parameters such as viral load and genotype among these cases, and determine markers associated with fatal outcome. Methods Acute severe YF cases (n = 62) were admitted to the Intensive Care Unit of a reference hospital and submitted to routine laboratorial evaluation on admission. YFV-RNA was detected in serum and urine by RT–qPCR and then sequenced. Patients were classified in two groups: survival or death. Results In the univariate analysis the following variables were associated with outcome: ALT, AST, AST/ALT ratio, total bilirubin, CKD-EPI, ammonia, lipase, factor V, INR, lactate, and bicarbonate. Logistic regression model showed two independent variables associated with death: lipase (OR 1.018, 95% CI 1.007 to 1.030, P = 0.002), and factor V (OR −0.955, 95% CI 0.929 to 0.982, P = 0.001). The estimated lipase and factor V cut-off values that maximized sensitivity and specificity for death prediction were 147.5 U/L (AUC = 0.879), and 56.5% (AUC = 0.913). Patients who were discharged from the hospital continued to be followed-up in the outpatient clinic. Seven patients had their urine and blood screened weekly for YFV until the test was negative. After the onset of symptoms, viremia and viruria were present for a maximum period of 28 days and 47 days, respectively. Conclusion YF acute severe cases show a generalized involvement of different organs (liver, spleen, heart, kidneys, intestines, and pancreas), and different parameters were related to outcome. Factor V and lipase are independent variables associated with death, reinforcing the importance of hemorrhagic events due to fulminant liver failure and pointing to pancreatitis as a relevant event in the outcome of the disease. Disclosures All authors: No reported disclosures.