Objective: Periodontitis may contribute to vascular damage, resulting in the destabilization of atherosclerotic plaque leading to acute coronary syndrome (ACS). In this study, we explored the effect of non-surgical periodontal treatment (NSPT) on cardiovascular blood biomarkers and gingival crevicular fluid (GCF) neutrophil elastase (NE) and α1-proteinase inhibitor (α-1PI) levels in periodontitis (P) participants with and without ACS. Materials and methods: Medical and dental examinations were performed to diagnose ACS and periodontitis, respectively. Seventeen patients with diagnosis of ACS and periodontitis were included in this study, as a test group (group ACS). Twenty-six age and sex-matched control patients with periodontitis (group P) were otherwise systemically healthy. Both groups received NSPT. Plasma levels of cholesterol, triglyceride, high-density lipoprotein (HDL), low-density lipoprotein (LDL), C-reactive protein (CRP), GCF NE activity, GCF α1-PI levels, and GCF NE/α1-PI rates were measured at baseline, at1st and 3rd months after NSPT. Results: GCF NE activity/time (μU/30s) decreased significantly at 3rd month compared to baseline values in the Group P after NSPT. First and 3rd months after NSPT, in the Group P GCF α1-PI activity/time (pg/30s) was significantly higher than the Group ACS. Moreover GCF NE/α1-PI rates decreased significantly compared to baseline values at 1st and 3rd months after NSPT in the group P. Conclusion: NSPT yields decrease in NE/α1-PI rates. NE and its possible interactions with α1-PI may play a crucial role in both periodontitis and ACS. GCF α1PI activity/time (U/30s) can be a potential biomarker in management of periodontitis associated with ACS. Clinical relevance: The GCF α1-PI reduction may alter the immune-inflammatory response in patients with periodontitis and thus increase the risk of ACS. Trial registration: Thai Clinical Trials.gov (NCT04785235)