Accumulated data on medical care utilization among the insured in Korea Medical Insurance Corporation can explain the health status of the population. The purpose of this study was to analyze a change of the disease-mix and utilization pattern by controlling the size of the population enrollment. Major findings of the study are as follows : 1. The changes of inpatient disease-mix a. Utilization rate was 139.2% in 1988 against 1980. b. Disease groups higher than the average utilization rate included neoplasms, endocrine, nutritional and metabolic diseases and immunity disorders, mental disorders etc. Meanwhile, disease groups seen less often were infections and parasistic diseases, diseases of blood and bloodforming, diseases of the digestive system etc. c. Utilization rate was up 106.3% in 1988 compared to 1985, and diseases above that average level were ill-defined intestinal infections, chronic liver disease and cirrhosis, diabetes mellitus, essential hypertension, etc. d. The disease-mix by institution in 1988 compared to 1985 shows that chronic disorders rank high in general hospitals whereas opthalmologic, obstetric, and orthopedic diseases rank high in private clinics. 2. The changes of outpatient disease-mix a. Utilization rate was up 175.2% in 1988 compared to 1980. b. Disease groups higher than the average utilization rate included neoplasms, endocrine, nutritional and metabolic diseases and immunity disorders, mental disorders etc. And disease groups seen less often were infections and parasistic diseases, diseases of the respiratory system, diseases of the genitourinary system. etc. c. Utilization rate was up 104.0% in 1988 compared to 1985, and diseases above that average level were gastric ulcer, diseases of hard tissues of teeth, etc. And diseases seen below that average level were acute nasopharyngitis(common cold). acute upper respiratory infections of multiple or unspecified sites, etc. It was concluded that medical care utilization level was increased, and that, from 1980 to 1988, disease-mix shifted to the chronic disorders. Chronic disorders accounted for more medical care utilization in general hospitals.