The advanced lung cancer inflammation index (ALI) is an independent prognostic biomarker of inflammation and nutrition in various types of cancer, acute heart failure and acute coronary syndrome. The SYNTAX (Synergy between Percutaneous Coronary Intervention with TAXUS and Cardiac Surgery) score (SXscore) is an angiographic scoring tool used to determine the extent and severity of coronary artery disease. To investigate the relationship between ALI and coronary artery lesion complexity assessed using the SXscore in patients with non-ST-segment elevation myocardial infarction (NSTEMI). Between February and November 2020, a total of 284 patients with NSTEMI were included consecutively. ALI was calculated with the formula body mass index (BMI) × serum albumin concentration/neutrophil-to-lymphocyte ratio (NLR). SXscore was calculated using the online calculator and divided into two groups - low (< 32) and high (≥ 33) - and then analyzed. Patients with a high SXscore had lower ALI (22.4 ±7.3 vs. 58.5 ±44.3, p = 0.016). In the univariable analysis, age (p = 0.046), BMI (p = 0.021), C-reactive protein (p = 0.002), peak troponin I (p = 0.009), NLR (p = 0.025), serum albumin (p = 0.003) and ALI (p < 0.001) were significantly associated with a high SXscore. ALI emerged as an independent predictor of a high SXscore in multivariable analysis (95% CI: 0.931-0.984, p = 0.002). The ALI may be useful as a simple tool for predicting high SXscore in patients with NSTEMI. To our knowledge, this is the first study to examine the relationship between ALI and severity of CAD.