['\u2002The Morning Hypertension and Angiotensin Receptor Blocker/Hydrochlorothiazide Combination Therapy (MAPPY) study has shown that losartan/hydrochlorothiazide (HCTZ) combination is superior to high-dose losartan in not only reducing morning systolic blood pressure (SBP) but also ameliorating urinary albumin excretion (UAE) after 3-month treatment. The purpose of the present study was to investigate factors associated with UAE reduction in on-treatment patients with morning hypertension.', '\u2002A total of 95 patients registered in the MAPPY study were analyzed. Patients were treated with either a losartan/HCTZ combination regimen (n=47) or a high-dose losartan regimen (n=48). Three-month treatment significantly reduced morning SBP, evening SBP, and clinic SBP (P<0.001, P<0.05, and P<0.01, respectively). UAE and serum uric acid were significantly decreased (P<0.01 for both) without the change in estimated glomerular filtration rate. Multiple linear regression analysis indicated that %morning SBP reduction and baseline UAE were independent determinants of the UAE reduction (P=0.001 for both). After adjustments for the reduction in morning-evening SBP difference, baseline UAE, and %uric acid reduction, estimated %UAE reduction level was positively correlated with the tertiles of the increasing %morning SBP reduction level (P=0.031 for trend). Moreover, subgroup analysis showed that morning SBP reduction was an independent determinant of UAE reduction in both treatment regimens.'] Is reduction in morning blood pressure a key factor for ameliorating urinary albumin excretion in patients with morning hypertension irrespective of treatment regimen?