Background: Prolonged fever is associated with adverse outcomes in dengue viral infection. Similar fever patterns are observed in COVID-19 with unclear significance. Methods: We conducted a hospital-based case-control study of patients admitted for COVID-19 with prolonged fever (fever >7 days) and saddleback fever (recurrence of fever, lasting <24 hours, after defervescence beyond day 7 of illness). Fever was defined as a temperature of >/=38.0 degrees C. Cytokines were determined with multiplex microbead-based immunoassay for a subgroup of patients. Adverse outcomes were hypoxia, intensive care unit (ICU) admission, mechanical ventilation, and mortality. Results: A total of 142 patients were included in the study; 12.7% (18/142) of cases had prolonged fever, and 9.9% (14/142) had saddleback fever. Those with prolonged fever had a median duration of fever (interquartile range [IQR]) of 10 (9-11) days for prolonged fever cases, while fever recurred at a median (IQR) of 10 (8-12) days for those with saddleback fever. Both prolonged (27.8% vs 0.9%; P < .01) and saddleback fever (14.3% vs 0.9%; P = .03) were associated with hypoxia compared with controls. Cases with prolonged fever were also more likely to require ICU admission compared with controls (11.1% vs 0.9%; P = .05). Patients with prolonged fever had higher induced protein-10 and lower interleukin-1alpha levels compared with those with saddleback fever at the early acute phase of disease. Conclusions: Prolonged fever beyond 7 days from onset of illness can identify patients who may be at risk of adverse outcomes from COVID-19. Patients with saddleback fever appeared to have good outcomes regardless of the fever.