Objective. The aim of this study was to investigate the relation between native thiol/disulfide ratio (TDR) and severity of coronary atherosclerosis as assessed by the Syntax score (SXscore) in patients with non-ST elevation myocardial infarction (NSTEMI) who underwent coronary angiography. Material and Methods. A total of 290 patients with NSTEMI who underwent coronary angiography, were included in the study between January and August 2014. Baseline coronary angiography determined the SXscore. The patients were divided into two groups: one with low SXscores (<23) and the other with high SXscores (>= 23). Results. TDR was significantly lower in patients with high SXscores (p < 0.001). In-hospital mortality was higher in the group with low TDR and high SXscores. The cut-off value of TDR on admission that predicted a high SXscore in the groups combined was 14, with a sensitivity of 73% and a specificity of 68%. Conclusion. TDR can be determined by an easy, inexpensive, automated, or optionally manual spectrophotometric assay, and correlates inversely with SXscore in patients with NSTEMI.