The Japan Hypertension Evaluation with Angiotensin II Antagonist Losartan Therapy (J-HEALTH) study is a nationwide, prospective, multicenter observational study that was designed to enroll hypertensive Japanese patients (>30,000 subjects). The patients in this study received treatment with open-label losartan, an angiotensin II receptor antagonist, for a maximum of 5 years. This report summarizes the study protocol and the baseline characteristics of the patients. Between June 2000 and May 2002, patients were screened in all 47 prefectures around Japan. Among the 31,515 patients screened, 31,048 patients were enrolled in this study and treated with losartan at a daily dose of 25-50 mg. These patients were 62.4 +/- 12.1 years old (mean +/- SD) and the mean clinic systolic/diastolic blood pressure (BP) values were 165.3 +/- 17.3/94.3 +/- 11.7 mmHg (mean +/- SD). The complications of hyperlipidemia, diabetes mellitus, cardiovascular disease, and cerebrovascular disease were also present in 38.5%, 13.1%, 8.0%, and 4.4% of patients, respectively. Regarding the World Health Organization classification, grade 2 hypertension was most frequent in this patient cohort. Nearly 10,000 patients agreed to perform home BP monitoring and report details regarding their lifestyles at baseline. Among the patients, 4.2% had white coat hypertension at the baseline. The J-HEALTH study is expected to provide valuable information about the significance of clinic and home BP control and home BP monitoring for the management of hypertension in Japanese patients.