BACKGROUND AND OBJECTIVES: Circulating endothelial progenitor cells (EPCs) play a key role in the maintenance of endothelial homeostasis and promote vascular repair. A reduced number of EPCs and the functional activity have been associated with several cardiovascular risk factors. However, the relationship between the number of EPCs and circadian rhythm of the blood pressure (BP) remains unclear. The purpose of the present study was to evaluate the relationship between the circadian rhythm of the BP and EPCs in patients with essential hypertension. SUBJECTS AND METHODS: A total of 45 patients with essential hypertension who were newly identified by outpatient BP measurements, underwent 24-hour ambulatory BP monitoring. Among the 45 patients with essential hypertension, 20 were classified as dippers (12 men and 8 women; mean age 48+/-14 years) and 25 as non-dippers (14 men and 11 women; mean age 52+/-18 years). The EPC count was isolated from the peripheral bloodstream and quantified by flow cytometry. RESULTS: The baseline clinical characteristics were similar between the dipper and non-dipper hypertensive patients. The circulating EPCs were statistically reduced in the non-dipper patients as compared to the dippers (104+/-60 vs. 66+/-47 EPCs per 106 mononuclear cells, p=0.027). The circulating EPC level correlated positively with the circadian changes in the systolic and diastolic BP (r=0.435, p=0.003, and r=0.310, p=0.038, respectively). CONCLUSION: The present study demonstrated that the EPC count was reduced in the peripheral bloodstream in non-dipper hypertensive patients.