INTRODUCTION: Hypertension is associated with microcirculatory disturbance, and erythrocyte deformability is a major determinant of the microcirculation. However, impairment of erythrocyte deformability in hypertensive patients in relation to antihypertensive treatment is unclear. The present study aimed to investigate this impairment in hypertensive patients under treatment using a highly sensitive and quantitative nickel mesh filtration technique. METHODS: Deformability was evaluated by filterability, defined as the flow rate of a hematocrit-adjusted erythrocyte suspension relative to that of saline under a specific filtration pressure in a pressure-flow curve obtained by continuous filtration. Baseline characteristics of hypertensive patients (n = 101) and age-matched normotensive subjects (n = 14) were obtained from medical records, and diabetic patients were excluded. RESULTS: Erythrocyte deformability in the hypertensive group was significantly (p = 0.010) lower (87.8 +/- 2.2 %) than that of the normotensive group (89.4 +/- 1.7 %) and inversely proportional (r = -0.303, p = 0.002) to the mean blood pressure (BP) measured on blood sampling for the filtration study. Stepwise multiple regression analysis demonstrated that this impairment was mostly attributable to the mean BP (p = 0.001), whereas current smoking and episodes of stroke or coronary artery disease were not contributors. DISCUSSION: These findings indicate that erythrocyte deformability is impaired in the hypertensive patients, which depends on the current BP control rather than target organ damage.