We assessed the association between serum cystatin C (CysC) levels and coronary slow flow (CSF) phenomenon in patients who underwent coronary angiography. A total of 210 patients (mean age 55.6 +/- 10.9 years; 100 females) were included. Patients were divided into 3 groups, (patients with CSF [group 2], with coronary artery disease [CAD; group 3], and without CAD [group 1]). Serum CysC levels in patients with CSF were significantly lower than those with and without CAD (912.5 +/- 135.6, 820.4 +/- 104.2, and 1343.4 +/- 236.6 ng/mL in groups 1, 2, and 3, respectively; P < .001). Serum CysC levels correlated with the number of vessels with CSF and mean corrected thrombolysis in myocardial infarction frame count (Spearman correlation coefficient [rs] = .192, P < .001 and rs = .261, P < .001 respectively). In conclusion, patients with CSF have lower CysC levels; this could be a useful biomarker of CSF involvement in patients who undergo diagnostic coronary angiography.