Previous studies have suggested that microcirculatory abnormalities cause slow coronary flow (SCF). However, the underlying mechanism of this phenomenon has not yet been well documented. Therefore, the aim of this study was to determine the role of plasma lipid disturbances in pathogenesis of slow coronary flow (SCF). Forty patients with SCF (group 1) and 37 subjects with normal coronary arteries (group 11) were included in the study. In each subject plasma lipid concentrations (total cholesterol [TC], low-density lipoprotein cholesterol [LDL-C], high-density lipoprotein cholesterol [HIDL-DC], and triglycericle [TG]) and brachial artery flow-mediated dilatation (FMD) and nitroglycerin (NTG)-induced dilatation were measured. Total cholesterol level was found to be similar in the 2 groups. In group 1, HDL-C level was lower than in group II (34 +/- 3 vs 40 +/- 4 mg/dL, p = 0.0001). In group I, TG level was higher than in group II (213 +/- 29 vs 198 +/- 24 mg/dL p = 0.002). In group I, FMD was smaller than that of group II (3.48 +/- 3.1% vs 10.4 +/- 5.6%, p=0.0001). The percent NTG-induced dilatation was not different between the groups (15.5 +/- 5.3%vs 17.3 +/- 6.9%, p=0.27). On regression analysis; there was a significant relationship between percent of FIVID and HDL-C (r=0.65, p=0.0001). When the 2 groups were analyzed separately, HDL-C was still related to percent of FMD in both groups (r=0.47 P=0.002 andr=0.45 p=0.005, respectively). Multivariate regression analysis showed that only plasma HDL-C was independently related to FIVID (F = 7.5 p = 0.0001). In patients with SCF, reduced flow-mediated dilatation was detected and was found to be associated with plasma lipid disturbances, principally low HDL and high TG levels.