We conducted a case-control study nested within a small cohort identified by using a hospital information system in Japan to examine the relationship of the use of calcium channel blockers (CCBs) to myocardial infarction. Although the crude odds ratio of myocardial infarction associated with CCBs was high, the ratio was reduced when adjusted by known confounding factors, suggesting a mechanism of confounding by indication. In addition, the results obtained in this study using records from a single hospital should not be generalized. Quality Indicators (QIs) are a tool for assessing healthcare quality, and QIs monitoring works to improve the quality of medical care. When QIs would be analyzed by using a hospital information, confounding factors should be removed if we measure QIs using the same method in the same hospital in chronological order. QIs analyzed by using a hospital information system are valuable and applicable to patients. It would be a driving force of the medical quality improvement that the number of the pharmaco-epidemiology experts would increase more and more. Because they can make the drug information conformed to Evidence-based Medicine using a medical database. Then, the medical quality improvement will decrease medical cost in Japan.