The Japan Hypertension Evaluation with Angiotensin II Antagonist Losartan Therapy (J-HEALTH) study is a nationwide, prospective, multicentered, observational study that was designed to enroll 30,000 hypertensive Japanese patients from more than 3,000 private practitioners. It is the first large-scale observational study to assess the efficacy and safety of losartan, an angiotensin II receptor antagonist, in Japan. Patients were enrolled between June 2000 and May 2002, and followed up to June 2005. The data from 29,850 patients were used for the analysis of safety and efficacy. These patients were treated with losartan mostly at a daily dose of 25-50 mg. The mean follow-up period was 2.9 years. The patients were aged 62.4+/-12.1 years (mean+/-SD) and their mean systolic/diastolic blood pressure was 165.3+/-17.2/94.3+/-11.7 mmHg (mean+/-SD). Mean blood pressure in patients who were evaluated for efficacy decreased from 165.8/94.8 mmHg (n=26,512) at baseline to 145.5/84.4 mmHg after 3 months (n=21,269) and 138.6/80.0 mmHg after 36 months of treatment (n=13,879). Blood pressure was well controlled during the study period by losartan alone or losartan-based combination therapy. In nearly half of the patients, blood pressure was reduced to less than 140/90 mmHg during the study period. In addition to its antihypertensive effect, losartan reduced the uric acid level in patients whose baseline uric acid level was > or =7 mg/dL. Losartan also prevented acceleration of proteinuria. Adverse drug reactions occurred in 1,081 of the 29,850 patients. Long-term losartan therapy was effective and well tolerated in Japanese clinical practice.