BACKGROUND Prolonged fever in pediatric patients is often a diagnostic challenge. Clinicians tend to associate prolonged fever with COVID-19-related diseases in patients with a history of SARS-CoV-2 infection. Here we present a patient who was admitted with a clinical suspicion of multi-inflammatory syndrome in children (MIS-C) and was finally diagnosed with a renal abscess. CASE REPORT A 16-year-old girl with prolonged fever, bilateral non-purulent conjunctivitis, weight loss, muscle pain, general malaise, cough, and yellow sputum was admitted to Heim Pál National Pediatric Institute, Budapest, Hungary. She had proven SARS-CoV-2 infection 3 weeks prior to admission. Although inflammatory markers were elevated, repeated urine analyses, aerobic and anaerobic urine cultures, hemoculture, chest X-ray, and otorhinolaryngology examinations were negative. Based on clinical and laboratory criteria, the diagnosis of MIS-C was eventually ruled out. Abdominal ultrasound revealed a 17×20×15 mm simplex cyst at the edge of the parenchyma in the upper third of the left kidney. Magnetic resonance imaging was performed, showing a multi-compartment, septated, thick-walled parenchymal lesion of 50×40×52 mm in the upper pole of the right kidney, which showed signal characteristics of an abscess, and 20×16 mm and 8 mm lesions in the upper pole of the left kidney, which appeared to be cysts. After being unresponsive to intravenous wide-spectrum antibiotic therapy (meropenem 2 g tid for 5 days), surgical intervention was needed to remove the abscess. CONCLUSIONS This case demonstrates that during the COVID-19 pandemic, besides the obvious post-COVID etiology, other life-threatening conditions should be investigated in the first line.