BACKGROUND AND OBJECTIVE: The complement system plays a crucial role in the pathogenesis and progression of cardiovascular diseases. C1q, a key initiator of the classical pathway, is closely associated with various chronic inflammatory conditions. This observational study aims to elucidate the potential risk relationship between serum complement C1q levels and atrial fibrillation (AF). MATERIALS AND METHODS: This retrospective cohort study included 812 AF patients treated at the Affiliated Hospital of Qingdao University from January 2020 to October 2022, comprising 694 patients in the paroxysmal AF group and 118 in the persistent AF group. Serum complement C1q levels were measured using an enzyme-linked immunosorbent assay (ELISA). RESULTS: Serum C1q levels in the AF group were significantly lower than those in the control group ( CONCLUSION: Serum C1q levels are significantly reduced in patients with AF. The presence of CFAE in the superior left atrium of paroxysmal AF patients may be potentially associated with C1q levels. Low complement levels are associated with atrial fibrillation compared to individuals without AF and may represent a potential underlying cause of impaired sinus rhythm maintenance following pulmonary vein isolation. Complement C1q may play a critical role in the pathogenesis of AF.