SUMMARY
A woman in her 50s with a history of stable ulcerative colitis for 20 years, managed using salazosulfapyridine, presented with migratory rashes, spiking fever, edema, and joint pain that started one week after receiving the BNT162B2 mRNA vaccine against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Laboratory tests revealed extremely high serum ferritin levels. The patient was diagnosed with adult-onset Still’s disease (AOSD) based on the relevant classification criteria after ruling out other diseases. Detection of high levels of interleukin-18, an inflammatory cytokine related to AOSD, supported the diagnosis. Non-steroidal anti-inflammatory drug monotherapy alone resulted in significant improvements of both the abovementioned symptoms and the elevated inflammatory marker levels. AOSD in a patient with ulcerative colitis is extremely rare. Only one case of AOSD with ulcerative colitis was reported before the coronavirus disease 2019 era. This case indicates that SARS-CoV-2 vaccination can trigger a hyperinflammatory response classified as AOSD in a patient with ulcerative colitis, which is extremely rare.