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Hypertension is both a cause and a consequence of kidney disease. Whether there is an association between the earliest stages of elevated blood pressure and variations in kidney function within the normal range in the general population has not been investigated using accurate methodology.Glomerular filtration rate (GFR) by iohexol clearance and 24-h blood pressure were measured in a cross-sectional sample (n = 1627) from the general population aged from 50 to 62 years. None of the participants had known cardiovascular disease, chronic kidney disease, or diabetes.In multiple linear regression analyses with multivariate adjustment, GFR was associated with both ambulatory SBP and DBP and their interaction in separate models for daytime and night-time (P < 0.05). For blood pressure in the normotensive range, GFR increased with higher daytime SBP and with night-time SBP and DBP. In the daytime, higher DBP was associated with a slight decrease in GFR.GFR was associated with blood pressure in both the normotensive and hypertensive range. Although no conclusion about causality can be drawn from these cross-sectional relationships, they are consistent with the hypotheses of a causal relationship between renal function and blood pressure at a very early stage of hypertension or renal impairment.