BACKGROUND QT interval prolongation can induce torsades de pointes (TdP), a potentially fatal ventricular arrhythmia. Recently, an increasing number of non-cardiac drugs have been found to cause QT prolongation and/or TdP onset. Moreover, recent findings have demonstrated the key roles of systemic inflammatory activation and fever in promoting long-QT syndrome (LQTS) and TdP development. CASE SUMMARY A 30-year-old woman was admitted with a moderate to high-grade episodic fever for two weeks. The patient was administered with multiple antibiotics after hospitalization but still had repeating fever and markedly elevated C-reactive protein. Once after a high fever, the patient suddenly lost consciousness, and electrocardiogram (ECG) showed transient TdP onset after frequent premature ventricular contraction. The patient recovered sinus rhythm and consciousness spontaneously, and post-TdP ECG revealed a prolonged QTc interval of 560 ms. The patient's clinical manifestations and unresponsiveness to the antibiotics led to the final diagnosis of adult-onset Still's disease (AOSD). There was no evidence of cardiac involvement. After the AOSD diagnosis, discontinuation of antibiotics and immediate initiation of intravenous dexamethasone administration resulted in the normal temperature and QTc interval. The genetic analysis identified that the patient and her father had heterozygous mutations in KCNH2 (c.1370C>T) and AKAP9 (c.7725A>C). During the 2-year follow-up period, the patient had no recurrence of any arrhythmia and maintained normal QTc interval. CONCLUSION This case study highlights the risk of systemic inflammatory activation and antibiotic-induced TdP/LQTS onset. Genetic analysis should be considered to identify individuals at high risk of developing TdP.