This paper presents the design and application of a clinical data warehouse. The clinical data warehouse is built based on past 10 years of clinical data of Seoul National University Hospital. We extracted the clinical data from the database of hospital information system database and the results of Health 21 Project', which is mainly composed of chief complaints, final diagnosis, discharge notes and survival data. By normalizing the terms of chief complaint, we are able to analyze the distribution patterns of symptoms within a diagnosis, and that of diagnosis related with a certain clinical symptom. Through our involvement with this project, we have learned that in order for a useful clinical data warehouse system, normalization and structured data entry is the key element of building clinical data warehouse.