Replacement of animal protein in the diet with soy protein is associated with decreased cholesterol levels. However, the effects of soy protein diet on endothelial function are not well known. The aim of the study was to investigate the effects of soy protein diet on plasma lipids and endothelial function parameters assessed by two different methods. Twenty hypercholesterolemic, non-smoker male patients (age 50± I 2) w ith a normal body mass index were inc luded. After calculating their daily requirements, a diet with 25-30% of energy from fats, 1 0-ı 2% from proteins and the rest from carbohydrates was instituted. Sixty percent of the animal source proteins of the diet were substituted by soy. The anthropometric measures, lipid parameters and endothelial functions of the subjects were assessed at baseline and 6 weeks after soy protein diet. Flow mediated endothelium dependent dilatation (EDD) and plasma thrombomodulin (TM) levels were evaluated as endothelial function parameters. Results: After diet plasma total cholesterol, lowdensity lipoprotein cholesterol, apolipoprotein B and triglyceride levels decreased significantly (pKeywords: Soy protein, hypercholesterolemia, endothelial function, thrombomodulin.