Aim: The most important cause of death in end-stage renal disease is cardiovascular diseases (CVDs). Endothelial dysfunction (ED) is one of the most important steps in the pathogenesis of CVD. Apelin is a peptide recently found to have important roles in cardiovascular and endothelial physiology; and we planned in our study to examine the relationship between apelin and endothelial functions in peritoneal dialysis (PD) patients. Methods: Forty-two patients who were followed up in the PD unit at Haseki Training and Research Hospital were involved. Endothelial functions were evaluated ultrasonographically by flow-mediated dilation (FMD) test. Apelin levels and routine laboratory tests were studied from the blood samples obtained from the patients. Results: There was a negative correlation between apelin and FMD percentage (r=-0.334, p=0.03). FMD percentage positively correlated with triglyceride and very low density lipoprotein levels (r=0.310, p=0.046; r=0.304, p=0.05, respectively). FMD percentage significantly positively correlated with systolic and diastolic blood pressure (r=0.565,p=<0.001; r=0.560,p=<0.001, respectively). FMD was significantly lower patients with hypertension, compared to those without hypertension (9±5.8 vs. 14.9±6.6; p=0.01). Conclusion: ED is closely related with hypertension and lipid profile in PD patients. Apelin may be used as a new marker for ED and a treatment alternative in PD patients.