['The baseline status of renin-angiotensin-aldosterone system (RAAS) might modify the blood pressure (BP) lowering effects of thiazide diuretics. This study aimed to determine if baseline RAAS indicated by serum aldosterone-to-renin ratio (ARR) could be associated with the add-on effects of thiazide on BP lowering in patients with other concomitant antihypertensive medication.', 'Non-diabetic hypertensive patients, either untreated or unsatisfactorily treated, were enrolled if their office systolic BP was ≥ 140 or diastolic BP ≥ 90 mmHg. After 2 weeks of diet control and lifestyle modification, patients with persistently elevated BP were prospectively given hydrochlorothiazide 50 mg daily for 2 weeks. Serum aldosterone-to-renin ratio (ARR) was determined before thiazide treatment. Patients with a significant (≥ 10%) reduction of office mean artery pressure (MAP) by thiazide treatment were defined as responders.', 'Among the 66 patients studied, 27 were defined as responders after a 2-week hydrochlorothiazide treatment. Baseline serum renin level was reduced and ARR increased (p = 0.009) in the responders as compared with the non-responders. A similar pattern was also apparent in patients with or without concomitant medications. Furthermore, baseline renin level was inversely and ARR positively correlated to the MAP reduction both in the whole patient group and in patients with concomitant medications. By stepwise multiple linear regression analysis, ARR was the only independent predictor for the response to thiazide treatment (β = 0.051, p = 0.007).'] Is baseline Serum Aldosterone-to-Renin Ratio Associated with the Add-on Effect of Thiazide Diuretics in Non-Diabetic Essential Hypertensives?