OBJECTIVES: To clarify whether or not the antihypertensive drug effect is proportional to the baseline pretreatment self-measured home blood pressure (HBP) in accordance with the law of initial value (Wilder's law). DESIGN: A post-hoc analysis of a multicentre clinical trial. SETTING: Outpatients across Japan with mild-to-moderate essential hypertension. PARTICIPANTS: Among 3518 randomised participants, 2423 who self-measured HBP during the pretreatment drug-free period (10-28 days after starting fixed-dose antihypertensive monotherapy) with a mean 7.0 years follow-up were eligible. MAIN OUTCOME MEASURES: We analysed individual HBP readings during pretreatment and monotherapy. RESULTS: The day-to-day HBP during both the pretreatment period and monotherapy period remains almost the same throughout each period; the results were consistent, regardless of the pretreatment HBP. Following monotherapy, the reduction in the HBP increased by 2.2 mm Hg (95% CI: 1.8 to 2.5 mm Hg) per 10 mm Hg pretreatment HBP increase, up to 11.0 mm Hg (95% CI: 9.9 to 12.0 mm Hg) among patients with an HBP ≥165 mm Hg during pretreatment. Among the 1005 patients receiving low-dose monotherapy (defined daily dose: 0.5 units), the reduction peaked at 8.9-9.1 mm Hg in those with pretreatment HBP 155-164 mm Hg and ≥165 mm Hg (p=0.88). CONCLUSIONS: According to Wilder's law, the HBP reduction due to fixed-dose monotherapy was proportional to the pretreatment HBP without any regression to the mean phenomenon. With low-dose antihypertensive drugs, however, the HBP reduction peaked in patients with a high pretreatment HBP, indicating the need for such patients to receive a sufficient amount of antihypertensive drug medication at the initial treatment. TRIAL REGISTRATION: UMIN Clinical Trial Registry (http://www.umin.ac.jp/ctr), Unique identifier: C000000137.