Abstract-Microalbuminuria is a known marker of subclinical organ damage. Its prevalence is higher in patients with resistant hypertension than in subjects with blood pressure at goal. On the other hand, some patients with apparently well-controlled hypertension still have microalbuminuria. The current study aimed to determine the relationship between microalbuminuria and both office and 24-hour ambulatory blood pressure. A cohort of 356 patients (mean age 64Ϯ11 years; 40.2% females) with resistant hypertension (blood pressure Ն140 and/or 90 mm Hg despite treatment with Ն3 drugs, diuretic included) were selected from Spanish hypertension units. Patients with estimated glomerular filtration rate Ͻ30 mL/min/1.73 m 2 were excluded. All patients underwent clinical and demographic evaluation, complete laboratory analyses, and good technical-quality 24-hour ambulatory blood pressure monitoring. Urinary albumin/creatinine ratio was averaged from 3 first-morning void urine samples. Microalbuminuria (urinary albumin/creatinine ratio Ն2.5 mg/mmol in males or Ն3.5 mg/mmol in females) was detected in 46.6%, and impaired renal function (estimated glomerular filtration rate Ͻ60 mL/min/1.73 m 2 ) was detected in 26.8%. Bivariate analyses showed significant associations of microalbuminuria with older age, reduced estimated glomerular filtration rate, increased nighttime systolic blood pressure, and elevated daytime, nighttime, and 24-hour diastolic blood pressure. In a logistic regression analysis, after age and sex adjustment, elevated nighttime systolic blood pressure (multivariate odds ratio, 1.014 [95% CI, 1.001 to 1.026]; Pϭ0.029) and reduced estimated glomerular filtration rate (multivariate odds ratio, 2.79 [95% CI, 1.57 to 4.96]; Pϭ0.0005) were independently associated with the presence of microalbuminuria. We conclude that microalbuminuria is better associated with increased nighttime systolic blood pressure than with any other office and 24-hour ambulatory blood pressure monitoring parameters. Key Words: resistant hypertension Ⅲ microalbuminuria Ⅲ urinary albumin/creatinine ratio Ⅲ ambulatory blood pressure monitoring Ⅲ night-systolic blood pressure Ⅲ estimated glomerular filtration rate I n the last 2 decades, microalbuminuria has risen consistently as a reliable marker of subclinical target organ damage, both in diabetic and nondiabetic persons. [1][2][3][4][5][6][7] It has been shown that microalbuminuria is a risk factor with significant prognostic impact for both incident cardiovascular and renal diseases and for all cause-mortality 1-4,6,8 -17. Current guidelines for the detection, prevention, and treatment of high blood pressure (BP) and chronic kidney disease have, therefore, included microalbuminuria as a determinant of cardiovascular and renal risk. 18,19 There is also some evidence of the association of high urinary albumin excretion (UAE) with elevated BP in subjects with resistant hypertension. 20 Until now, 24-hour ambulatory BP monitoring (ABPM) has been the best-known tool to identify patients with true...