A dysregulated inflammatory response contributes to decline in patients with COVID-19. This cross-sectional study evaluated biomarkers of unvaccinated patients admitted to the intensive care unit of a hospital in Fortaleza, Brazil. Twenty cytokines were quantified upon hospital admission; clinical and laboratory data were analyzed, as well as sociodemographic data, to search for an association with clinical outcomes, including fatal (n = 40) or recovered cases (n = 38). Fatal cases exhibited significantly higher levels of IL-18 (p = 0.009); deceased patients were older (p = 0.0001), had a lower number of platelets (p = 0.0063) and higher neutrophil-lymphocyte ratio (p = 0.0230) than those who recovered. These findings indicate that IL-18 is a possible marker to predict poor prognosis in critically ill patients with COVID-19.