OBJECTIVE: An elevated red cell distribution width has been recognized as a predictor of various cardiovascular diseases. Slow coronary flow syndrome is an important angiographic clinical entity with an unknown etiology. This study aimed to examine the relationship between red cell distribution width and the presence of slow coronary flow syndrome. METHODS: In total, 185 patients with slow coronary flow syndrome and 183 age- and gender-matched subjects with normal coronary flow (controls) were prospectively enrolled in this study. Red cell distribution width and C-reactive protein were measured upon admission, and the results were compared between the patients with slow coronary flow syndrome and normal controls. RESULTS: Red cell distribution width levels were significantly higher in the patients with slow coronary flow syndrome than the normal controls. Moreover, the data showed that the plasma C-reactive protein levels were also higher in the patients with slow coronary flow syndrome than in the normal controls. In addition, a multivariate analysis indicated that C-reactive protein and red cell distribution width were the independent variables most strongly associated with slow coronary flow syndrome. Finally, the red cell distribution width was positively correlated with C-reactive protein and mean thrombosis in the myocardial infarction frame counts of the patients with slow coronary flow syndrome. CONCLUSION: The data demonstrated that red cell distribution width levels are significantly higher and strongly positively correlated with both C-reactive protein and thrombosis in the myocardial infarction frame counts of patients with slow coronary flow syndrome. These findings suggest that red cell distribution width may be a useful marker for patients with slow coronary flow syndrome.