['In patients with essential hypertension (EHT), the intrarenal resistance index (RI) has been shown to be related to the severity of target organ damage (TOD). Cystatin C is has been reported to be related to TOD in EHT. The aim of the present study was to clarify whether the RI predicts future renal function assessed by cystatin C levels in EHT.', 'One-hundred and twelve patients participated. RI and cystatin C were measured at baseline, and 12 months later, cystatin C was measured again.', 'The patients were divided into 2 groups according to RI value: the low RI group (RI<0.7) and the high RI group (RI> or =0.7). After 12 months, cystatin C levels were significantly elevated in the high RI group, whereas the levels remained unchanged in the low RI group. Stepwise regression analysis using the baseline values of RI, age, pulse pressure, HbA1c, cystatin C, log-transformed (ln) C-reactive protein and ln urinary albumin/creatinine as covariates, showed baseline RI was the only independent determinant of the time-related changes in cystatin C levels.'] Is renal resistance index a marker of future renal dysfunction in patients with essential hypertension?