Background: Slow flow velocity of dye in coronary arteries is not an infrequent finding during routine coronary angiography and its precise mechanism is unknown. In this study, we measured the plasma homocysteine level in patients with slow coronary flow (SCF) in comparison with subjects having normal coronary flow (NCF). Method: The study consisted 39 patients (mean age, 47 +/- 8 years) with angiographically diagnosed SCF. SCF was defined according to TIMI frame count (TFC) method. Thirty subjects (mean age 46 8 years) with NCF served as control group. Plasma homocysteine levels were measured after 12 h fasting period in each subject. Results: Baseline demographic properties were similar in both groups. In patients with SCF, TFC was significantly higher than those with NCF. Similarly, in patients with SCF plasma homocysteine level was significantly higher than that of control group (14.1 +/- 2.2 vs. 5.5 +/- 1.3 mu mol/l, respectively p < 0.001). Conclusion: Elevated plasma homocysteine level supports the hypothesis that endothelial function is impaired in slow coronary flow. (c) 2004 Elsevier Ireland Ltd. All rights reserved.