Background: To explore the relationship between coronary slow flow (CSF) and both matrix metalloproteinase-9 (MMP-9) and soluble cluster of differentiation 40 ligand (sCD40L).Methods: Between July 2016 and September 2019, 80 patients suspected of having coronary heart disease due to chest tightness or other symptoms of chest pain and discomfort were admitted to Nanjing Jiangbei People's Hospital and, following a coronary angiography, were diagnosed as having CSF. These patients were assigned to the coronary slow flow phenomenon (CSFP) group. The control group, or normal coronary flow (NCF) group, consisted of 80 patients with normal coronary angiography results. Following group assignment, enzyme-linked immunosorbent assays (ELISAs) were used to measure and compare the serum levels of sCD40L and MMP-9 between the two groups.Results: Serum levels of sCD40L and MMP-9 in the CSFP group were significantly higher than those in the NCF group (P<0.05). Additionally, serum levels of adhesion molecules and C-reactive protein (CRP) in the CSFP group were significantly higher than those in NCF group (P<0.05). There was a positive correlation observed between sCD40L levels and adhesion molecule levels in the CSFP group, however, there was no such correlation observed in the NCF group. The CSFP group also had a significantly higher corrected TIMI frame count (CTFC) compared with the NCF group (P<0.05).Conclusions: Serum MMP-9 levels were higher in patients with CSF, and MMP-9 levels were positively correlated with CTFC. This suggests that chronic inflammation may play a role in CSF pathogenesis and that atherosclerotic plaques may be present in the coronary arteries of CSF patients. Additionally, serum levels of sCD40L, adhesion molecules, and CRP were higher in CSF patients, and CRP levels were positively correlated with CTFC. Adhesion molecule expression is promoted by sCD40L. Increased levels of sCD40L may therefore promote the development of CSF.