Purpose: In Japan, all patients are able to see freely any clinics or hospitals. So clinical data of all patients have been stored at clinics, hospitals and medical institutes respectively. These patients' clinical course data stocks have not been combined with one another. There is no large-scale database, which has been available and has played its role in complementing spontaneous adverse drug reaction (ADR) reporting system. We tried to build an original database using anti-hypertensive drugs' data from Drug Use Investigation conducted for the Japanese Drug Re-examination application by every pharmaceutical manufacturer in conformity with Japanese Pharmaceutical Affairs Law and Related Regulation., Methods: The 43 565 case data of seven anti-hypertensive drugs (one Ca-antagonist, one alpha-blocker, two beta-blockers, three ACE inhibitors) were kindly offered from seven manufacturers who were members of RAD-AR Council, Japan. After examining the data items and categories, they were standardized into common codes based on Japanese Drug Category Classification (JDCC), International Classification of Diseases 9 (ICD-9) and Japanese Adverse Drug Reaction Terminology (J-ART). As each manufacturer had a different coding method in accordance to manufacturer's own practice of data management, the original forms were divided into several datasets. The data processing and statistical analysis were conducted using Statistical Analysis System (SAS)., Results: (1) Technology and know-how to combine data coded by different methods were established for building a database that had never been tried in Japan. (2) The following are the by-products of the study: a) Onset of ADR concentrated in the early stage but onset of some disorders prevailed equally throughout the investigation period. b) Although the number of collected cases of anti-hypertensive drugs was 43 565, total number of administrated anti-hypertensive drugs reached to 70 714 because additional anti-hypertensive drugs were often used with subject drugs., Conclusion: There is no large-scale database of patients' clinical course in Japan. However, since the Japanese Drug Re-examination System started in 1979, almost eight million cases of Drug Use Investigation on about 700 drugs have been collected with enormous human power and huge expenditure for Japanese Drug Re-examination application by pharmaceutical manufacturers. New and more appropriate information will be detected by the database, built using Drug Use Investigation data that were collected only for the Japanese Drug Re-examination application., (Copyright (c) 2004 John Wiley & Sons, Ltd.)