On the basis of the studies that investigated the relationship between baseline clinic blood pressure (CBP) or home blood pressure (HBP) values and cardiovascular (CV) events, HBP has been reported to have a stronger prognostic ability. However, few studies have compared the prognostic ability of on-treatment CBP and HBP. The relationship between on-treatment HBP, measured twice in the morning and twice at bedtime, and CV events was investigated in over 20 000 patients in the HONEST (Home blood pressure measurement with Olmesartan Naive patients to Establish Standard Target blood pressure) Study, a prospective, 2-year observational study of treatment with an angiotensin receptor blocker, olmesartan (OLM), in OLM-naive hypertensive patients. This report summarizes the study protocol, the baseline characteristics of the patients and CBP and HBP at 16 weeks. A total of 22 373 patients were registered across Japan; baseline data from 22 162 patients were collected. Baseline HBP (mean±s.d.) in the morning (the first measurement) was 151.6±16.4/87.1±11.8 mm Hg and at bedtime was 144.3±16.8/82.8±11.9 mm Hg, whereas CBP was 153.6±19.0/87.1±13.4 mm Hg. At 16 weeks, morning HBP was 135.0±13.7/78.8±9.9 mm Hg and bedtime HBP was 129.7±13.8/74.7±10.1 mm Hg, whereas CBP was 135.6±15.4/77.6±10.9 mm Hg. The follow-up period for each patient ends on 30 September 2012. The HONEST Study is expected to provide evidence showing the relationship between baseline and on-treatment CBP and HBP levels (both first and second measurements) and CV events.